fb: fix warning: no return statement in function returning non-void

Warning is reproducible with selected FB_CFB_REV_PIXELS_IN_BYTE.

  CC      drivers/video/sysfillrect.o
In file included from drivers/video/sysfillrect.c:18:
drivers/video/fb_draw.h: In function `fb_rev_pixels_in_long':
drivers/video/fb_draw.h:94: warning: no return statement in function returning non-void
  CC      drivers/video/syscopyarea.o
In file included from drivers/video/syscopyarea.c:22:
drivers/video/fb_draw.h: In function `fb_rev_pixels_in_long':
drivers/video/fb_draw.h:94: warning: no return statement in function returning non-void

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>