mwifiex: use return value of mwifiex_add_virtual_intf() correctly

mwifiex_add_virtual_intf() returns ERR_PTR values. So use IS_ERR()
macro instead of checking for NULL pointer.

Reported-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>