rtlwifi: rtl8821ae: Convert inline routines to little-endian words

In this step, the read/write routines for the descriptors are converted
to use __le32 quantities, thus a lot of casts can be removed. Callback
routines still use the 8-bit arrays, but these are changed within the
specified routine.

The macro that cleared a descriptor has now been converted into an inline
routine.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>