mwifiex: use get_random_mask_addr() helper

Avoid calculating random MAC address in driver. Instead make
use of 'get_random_mask_addr()' function.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>