[media] as102: fix potential double free in as102_fw_upload()

In case the request to locate the firmware file part 2 fails, the error
path releases the already freed firmware memory location again. Thus
reset the firmware pointer to NULL after releasing firmware file part 1.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>