[media] staging/media/as102: Don't call release_firmware() on uninitialized variable

If, in drivers/staging/media/as102/as102_fw.c::as102_fw_upload(), the call
	cmd_buf = kzalloc(MAX_FW_PKT_SIZE, GFP_KERNEL);
should fail and return NULL so that we jump to the 'error:' label,
then we'll end up calling 'release_firmware(firmware);' with
'firmware' still uninitialized - not good.

The easy fix is to just initialize 'firmware' to NULL when we declare
it, since release_firmware() deals gracefully with being passed NULL
pointers.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>