[media] wl128x: fix int type for streg_cbdata

The streg_cbdata can have a negative error value. So, it should be
an integer, and not u8, as reported by smatch:
	drivers/media/radio/wl128x/fmdrv_common.c:1517 fmc_prepare() warn: assigning (-115) to unsigned variable 'fmdev->streg_cbdata'

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>