mwifiex: fix 5GHz association issue

Sometimes association in 5GHz doesn't work. Dmesg log shows
"Can not find requested SSID xyz" error message. Currently
while preparing scan channel list for firmware Null entries
are created for disabled channels. The routine which retrieves
this list ignores channels after Null entry. Hence sometimes
driver doesn't scan the channel of requested AP and association
fails. The issue is fixed by avoiding those NULL entries.

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>