staging: sm750fb: fix off-by-one in lynxfb_ops_setcolreg

staging: sm750fb: fix off-by-one in lynxfb_ops_setcolreg

The bounds check used regno > 256 instead of regno >= 256,
allowing regno == 256. Valid indices are 0–255, resulting
in an out-of-bounds write.

Also remove the regno < 256 check in the truecolor path,
as it is always true with the corrected guard.

Signed-off-by: Ahmet Sezgin Duran <ahmet@sezginduran.net>
Link: https://patch.msgid.link/20260408181210.9672-1-ahmet@sezginduran.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>