mwifiex: remove list traversal in mwifiex_num_pkts_in_txq

Instead of counting the number of packets in txq
for particular RA list before AMSDU creation,
maintain a counter which will keep track of the
same.

This will reduce some MIPS while generating AMSDU
traffic as we only have to check the counter instead
of traversing through skb list.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>