mwifiex: do not declare wdev as pointer

wdev is used even after del_virtual_interface handler in cfg80211
in nl80211_post_doit. Since we have freed wdev in handling of
del_virtual_intf, this can result into crash while deleting
interface.
Avoid this be not declaring wdev which part of
mwifiex_private structure but struct wireless_dev type.

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>