Staging: sm750fb: provide error path for hw_sm750le_setBLANK()

This provides a default path for the switch statement in
hw_sm750le_setBLANK() so that the compiler will not correctly complain
about undefined values being sent to the hardware.

Instead, properly error out if the blank command is unknown by the
driver.

Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/sm750fb/sm750_hw.c

@@ -472,6 +472,8 @@ int hw_sm750le_setBLANK(struct lynxfb_output * output,int blank){
 			dpms = CRT_DISPLAY_CTRL_DPMS_3;
 			crtdb = CRT_DISPLAY_CTRL_BLANK_ON;
 			break;
+		default:
+			return -EINVAL;
 	}
 
 	if(output->paths & sm750_crt){