staging: comedi: ni_tio: remove BUG() in ni_tio_set_gate_src()

This BUG() can never happen. The previous ni_tio_has_gate2_registers()
check will have already caused the function to return -EINVAL.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_tio.c

@@ -1020,8 +1020,7 @@ int ni_tio_set_gate_src(struct ni_gpct *counter,
 		case ni_gpct_variant_660x:
 			return ni_660x_set_gate2(counter, src);
 		default:
-			BUG();
-			break;
+			return -EINVAL;
 		}
 		break;
 	default: