drivers: led: is31fl3194.c fix uninitialized scalar variable

in the switch case in the preceeding for loop if default path is taken
all the time, then the ret variable will stay uninitialized, the original
contributor of this driver has provided a comment that this path shall
never be reached, however, it is better to return an error code instead
of continuing with an incorrect configuration, hence this commit replaces
continue with a proper return errno.

Signed-off-by: Jilay Pandya <jilay.pandya@outlook.com>

drivers/led/is31fl3194.c

@@ -109,7 +109,7 @@ static int is31fl3194_set_color(const struct device *dev, uint32_t led, uint8_t
 			break;
 		default:
 			/* unreachable: mapping already tested in is31fl3194_check_config */
-			continue;
+			return -EINVAL;
 		}
 
 		ret = i2c_reg_write_byte_dt(&config->bus, led_channels[i], value);