mwifiex: failure path handling in mwifiex_add_virtual_intf()

1) If register_netdevice() is failed, we are freeing netdev
pointer, but priv->netdev is not cleared. This gives kernel
paging request error when driver is unloaded or interface is
deleted. Fix the problem by clearing the pointer.
2) Fix memory leak issue by freeing 'wdev' in failure paths.
Also, clear priv->wdev pointer.

As mwifiex_add_virtual_intf() successfully handles the
failure conditions, redundant code under err_add_intf label
is removed in this patch.

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>