mwifiex: block scan request during heavy Tx traffic

Currently scan operation is delayed/aborted based on Tx traffic
consistency. This decision is taken after receiving scan
response of first scan command from FW. But when heavy traffic
is running, we can not even afford to send first scan command
and go off channel for 30msec. We will block scan request in
this case.

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>