rtw88: add support for set/get antennas

User space program such as iw can set antenna mask for the device.
So add set antenna support by configure the trx mode.

This is useful for some tests want to see the output of different
antenna configuration (e.g. path A v.s. path B).

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200410100950.3199-3-yhchuang@realtek.com