rtlwifi: Support A-MSDU in A-MPDU capability

Due to the fact that A-MSDU deaggregation is done in software,
we set this flag to support the A-MSDU in A-MPDU

Signed-off-by: Steven Ting <steventing@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>