mwifiex: add cfg80211 handlers add/del_virtual_intf

Making adding and deleting virtual interfaces dynamic. Adding
handlers for creating and deleting virtual interface with
given name and dev respectively.

Also, creating default interface of type station on insmod of
the driver.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>