mwifiex: fix typo in del_virtual_intf() function

Interface counters should be incremented in add_virtual_intf()
and decremented in del_virtual_intf() function. This patch
corrects it.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>