mwifiex: do not decrement rx_pending count twice

This patch fixes an issue with rx processing function where we were
decrementing rx_pending count twice.

Patch also queues correct main_work item when rx_pending count reaches
low watermark.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>