mwifiex: fix wrong return values in add_virtual_intf() error cases

add_virtual_intf() needs to return an ERR_PTR(), instead of NULL,
on errors, otherwise cfg80211 will crash.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>