mwifiex: invalidate bss config before setting channel for uAP

Mark bss_config parameters as invalid before setting AP channel.
This prevents from setting invalid parameters while setting AP
channel to FW.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>