base-files/system.sh: remove $ in $(())

Not needed.

https://github.com/koalaman/shellcheck/wiki/Sc2004

Signed-off-by: Rosen Penev <rosenp@gmail.com>

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

@@ -140,7 +140,7 @@ macaddr_add() {
 	local oui=${mac%:*:*:*}
 	local nic=${mac#*:*:*:}
 
-	nic=$(printf "%06x" $((0x${nic//:/} + $val & 0xffffff)) | sed 's/^\(.\{2\}\)\(.\{2\}\)\(.\{2\}\)/\1:\2:\3/')
+	nic=$(printf "%06x" $((0x${nic//:/} + val & 0xffffff)) | sed 's/^\(.\{2\}\)\(.\{2\}\)\(.\{2\}\)/\1:\2:\3/')
 	echo $oui:$nic
 }