mwifiex: update max_chan_per_scan correctly for SSID scan

As per recent patch "658f37b7 mwifiex: scan less channels..."
less channels are scanned per scan command in associated state.

Default number of channels per scan command for normal scan are
already 4, but those are 14 for SSID specific scan operation.

This code change in this patch is required for SSID specific scan.

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>