mx3fb: Fix print format string

%ul was probably meant as %lu since the former would print
an unsigned value and a letter l.

But in fact the whole value we are printing in u32 anyway, so
we don't need the format to be long. Therefore just drop the l
altogether.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>