base-files: minor fix to mmc_get_mac_ascii function

This is mostly a cosmetic cleanup. The absence of
the return statement was not causing any problems.

Signed-off-by: Rodrigo Balerdi <lanchon@gmail.com>
(cherry picked from commit 8cf4ac5195)

package/base-files/files/lib/functions/system.sh

@@ -205,6 +205,7 @@ mmc_get_mac_ascii() {
 	part=$(find_mmc_part "$part_name")
 	if [ -z "$part" ]; then
 		echo "mmc_get_mac_ascii: partition $part_name not found!" >&2
+		return
 	fi
 
 	get_mac_ascii "$part" "$key"