mwifiex: fix a possible double free issue

As drv_info_dump pointer doesn't get reset, we may end up
freeing the allocated memory twice.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>