rtw88: increse the size of rx buffer size

The vht capability of MAX_MPDU_LENGTH is 11454 in rtw88; however, the rx
buffer size for each packet is 8192. When receiving packets that are
larger than rx buffer size, it will leads to rx buffer ring overflow.

Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200925061219.23754-2-tehuang@realtek.com