mwifiex: manage virtual interface limits efficiently

Currently interface limits are checked by seeing if bss_mode for
particular priv is set. If bss_mode is not set, interface creation
is allowed. This patch adds framework to initializes maximum virtual
interfaces supported during load time and check current number of
interfaces created agains allowed interface limit during new virtual
interface creation.

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