sm501fb: fix timing limits

Vertical sync height register can only hold 6 bits.  Fix the hsync start test
to use > instead of >=.  Also add a few clarifying comments.

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>