mwifiex: fix leak of gen_ie storage on exit from mwifiex_del_mgmt_ies

Storage pointed to by gen_ie is allocated with kmalloc, but was
never freed.

Coverity CID #1271251

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>