rtlwifi: Reduce IO in RX interrupt to boost throughput

Check remaining count of RX packets cost a lot of CPU time, so only update
when the counter decreases to zero. In old flow, the counter was updated
once a RX packet is received.

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>