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 <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/sh7760fb.c

···
 		break;
 	case LDDFR_4BPP_MONO:
 		lgray = 1;
+		fallthrough;
 	case LDDFR_4BPP:
 		lbpp = 4;
 		break;
 	case LDDFR_6BPP_MONO:
 		lgray = 1;
+		fallthrough;
 	case LDDFR_8BPP:
 		lbpp = 8;
 		break;