mwifiex: maintain outstanding packet count for RA list instead of packet size

Maintain total outstanding packet count for RA list instead of total
outstanding size as packet count metric seems more reasonable for
checking threshold etc.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>