mwifiex: fix rx_pending count imbalance

RX packets are handled in different paths. Not all paths have
decrement of rx_pending counter. This patch fixes the counter
imbalance.

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>