OMAPFB: prevent buffer underflow in omapfb_parse_vram_param()

We cap the upper bound of "fbnum" but we also need to check for
negatives or make the type unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>