Openwrt and new compiler related fixes (#285)

* rtw-ap: Fix compiler warning.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* makefile:  Add entry for compiling against openwrt tree.

Users would need to edit it to match their path, but
this should get them started.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* makefile:  Allow defining KSRC outside of build.

So, you can do this:

KSRC=/tmp/my/kernel make

And not have to edit the makefile to have this function as desired.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Fix stack-too-large warning on x86-64 compile

Signed-off-by: Ben Greear <greearb@candelatech.com>

* compile:  Allow cross-compiling on cmd-line w/out editing Makefile

For instance:

KSRC=/home/greearb/git/openwrt-neo2-dev/build_dir/target-aarch64_cortex-a53_musl/linux-sunxi_cortexa53/linux-4.14.78 \
 EXT_EXTRA_CFLAGS=-DCONFIG_LITTLE_ENDIAN ARCH=arm64 CROSS_COMPILE=aarch64-openwrt-linux- MODDESTDIR=/tmp make V=1

Signed-off-by: Ben Greear <greearb@candelatech.com>

* build:  Attempt to auto-detect endian-ness.

Will make building in openwrt much easier.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Fix build against openwrt backports tree.

Like breaks builds elsewhere, can fix it up later.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Register wiphy after we probe MAC addr.

This way the phy object has a valid MAC-addr, which can be helpful
for identification.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Make sure MAC is set in wiphy_preinit

This is needed to make sure the phy registers with a proper MAC
address instead of all 00

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Fix build on 4.19 kernel.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Fix compile against 4.20 kernel.

Remove some variable-length arrays (which could be security bugs,
ways to overflow the stack it seems), and remove use of
get_monotonic_boottime.

Signed-off-by: Ben Greear <greearb@candelatech.com>

* Support arm64

include/wifi.h

@@ -977,7 +977,7 @@ typedef enum _HT_CAP_AMPDU_FACTOR {
  * According to IEEE802.11n spec size varies from 8K to 64K (in powers of 2)
  */
 #define IEEE80211_MIN_AMPDU_BUF 0x8
-#define IEEE80211_MAX_AMPDU_BUF 0x40
+//#define IEEE80211_MAX_AMPDU_BUF 0x40
 
 
 /* Spatial Multiplexing Power Save Modes */