sm501fb: set transp.offset to 0 in 8bpp and 16bpp modes

Even though it may not be strictly necessary transp.offset should probably be
0 when alpha channel is not available.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Acked-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>