linux-firmware: menu entries for mwifiex blobs

Firmware blobs for Marvell WiFi-Ex 8797, 8887, 8897 were already
there in linux-firmware tree but relevant options were not in Config.in.

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>