mwifiex: correct bss_type assignment

Correct bss_type assignment in add_virtual_interface.
This would ensure correct operation in multiple station scenarios.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>