fbdev: sh7760fb: Fix -Wimplicit-fallthrough warnings

Fix the following fallthrough warnings seen after building sh
architecture with sh7763rdp_defconfig configuration:

drivers/video/fbdev/sh7760fb.c: In function 'sh7760fb_get_color_info':
drivers/video/fbdev/sh7760fb.c:138:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
  138 |                 lgray = 1;
      |                 ~~~~~~^~~
drivers/video/fbdev/sh7760fb.c:139:9: note: here
  139 |         case LDDFR_4BPP:
      |         ^~~~
drivers/video/fbdev/sh7760fb.c:143:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
  143 |                 lgray = 1;
      |                 ~~~~~~^~~
drivers/video/fbdev/sh7760fb.c:144:9: note: here
  144 |         case LDDFR_8BPP:
      |         ^~~~

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>