mwifiex: improve scan delay logic during Tx traffic

Earlier scan command was postponed by 20msec multiple times to give
preference to Tx traffic until we find empty wmm queue.

There is a corner case in which wmm queue becomes empty immediately
after processing the packet(before 20msec) and there may be next
packet coming after some time. In this case we should not resume scan
operation.

We will use new flag to check Tx traffic and resume scan operation
only if there is no traffic for 200msec.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>