drivers: video: fbcmap: remove the redundency and incorrect checkings

fb_set_cmap() already checks the parameters, so need remove the
redundancy checking.

This redundancy checking is also incorrect, the related warning:

  drivers/video/fbcmap.c:288:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>