[ath_hal] [ath_hal_ar9300] Fix endian macros to work in and out of kernel tree.

Yes, people shouldn't use bitfields in C for structure parsing.
If someone ever wants a cleanup task then it'd be great to remove them
from this vendor code and other places in the ar9285/ar9287 HALs.

Alas, here we are.

AH_BYTE_ORDER wasn't defined and neither were the two values it could be.
So when compiling ath_ee_print_9300 it'd default to the big endian struct
layout and get a WHOLE lot of stuff wrong.

So:

• move AH_BYTE_ORDER into ath_hal/ah.h where it can be used by everyone.
• ensure that AH_BYTE_ORDER is actually defined before using it!

This should work on both big and little endian platforms.