[media] fmdrv: Don't check if unsigned are below zero

drivers/media/radio/wl128x/fmdrv_common.c:745:3: warning: comparison is always false due to limited range of data type [-Wtype-limits]
drivers/media/radio/wl128x/fmdrv_rx.c:308:2: warning: comparison is always false due to limited range of data type [-Wtype-limits]

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