OMAPFB: fix parsing of vram parameter

omapfb_parse_vram_param()'s check for end pointer returned from
simple_strtoul() is wrong, causing the code to bug if the second or
later vram parameters are non-parseable, for example
"omapfb.vram=0:2M,:5M".

However, even in that case the code will most likely bail out with
-EINVAL in the following checks, so the bug is probably not a fatal one.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Hein Tibosch <hein_tibosch@yahoo.es>