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>

drivers/video/fbdev/omap2/omapfb/omapfb-main.c

@@ -1477,7 +1477,7 @@ static int omapfb_alloc_fbmem_display(struct fb_info *fbi, unsigned long size,
 static int omapfb_parse_vram_param(const char *param, int max_entries,
 		unsigned long *sizes, unsigned long *paddrs)
 {
-	int fbnum;
+	unsigned int fbnum;
 	unsigned long size;
 	unsigned long paddr = 0;
 	char *p, *start;