base-files: fix mtd_get_mac_text not accepting hex offsets

The mtd_get_mac_text helper method did not support hexadecimal offset
values, resulting them to break after 75bfc393ba ("treewide:
convert MAC address location offsets to hexadecimal")

This commit fixes this by evaluating the hexadecimal input,
converting them to decimal.

Signed-off-by: David Bauer <mail@david-bauer.net>

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

@@ -40,7 +40,7 @@ mtd_get_mac_ascii() {
 
 mtd_get_mac_text() {
 	local mtdname=$1
-	local offset=$2
+	local offset=$(($2))
 	local part
 	local mac_dirty