mwifiex: limit channel number not to overflow memory

Limit the channel number in scan request, or the driver scan
config structure memory will be overflowed.

Cc: <stable@vger.kernel.org> # 3.5+
Signed-off-by: Stone Piao <piaoyun@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>