mwifiex: scan less channels per scan command to improve Tx traffic

Currently 4 channels are scanned per scan command. if scan request
is issued by user during Tx traffic, radio will be out of channel
for "4 * per_chan_scan_time" for each scan command and will not be
able to receive Rx packets. This adds delay in data traffic. We can
minimize it by reducing number of channels scanned per scan command
in this scenario.

We can not always scan 1 channel per scan command due to limitation
of number of command buffers. So we add code to decide number of
channels scanned per scan command in associated state.

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>