mwifiex: do not free firmware dump memory in shutdown_drv

mwifiex_upload_device_dump() already takes care of freeing firmware dump
memory. Doing the same thing in mwifiex_shutdown_drv() is redundant.

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