rtw88: pci: define a mask for TX/RX BD indexes

Add a macro TRX_BD_IDX_MASK for access the TX/RX BD indexes.

The hardware has only 12 bits for TX/RX BD indexes, we should not
initialize a TX/RX ring or access the TX/RX BD index with a length
that is larger than TRX_BD_IDX_MASK.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200312080852.16684-5-yhchuang@realtek.com