base-files: remove unneeded '$' signs in nand.sh

When using Shell arithmetric evaluation via $((..)) the variables in
the expression do not need to be prefixed by the '$' sign.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/base-files/files/lib/upgrade/nand.sh

@@ -118,7 +118,7 @@ nand_upgrade_prepare_ubi() {
 	local rootfs_length="$1"
 	local rootfs_type="$2"
 	local rootfs_data_max="$(fw_printenv -n rootfs_data_max 2>/dev/null)"
-	[ -n "$rootfs_data_max" ] && rootfs_data_max=$(($rootfs_data_max))
+	[ -n "$rootfs_data_max" ] && rootfs_data_max=$((rootfs_data_max))
 
 	local kernel_length="$3"
 	local has_env="${4:-0}"
@@ -195,7 +195,7 @@ nand_upgrade_prepare_ubi() {
 	if [ "$rootfs_type" != "ubifs" ]; then
 		local availeb=$(cat /sys/devices/virtual/ubi/$ubidev/avail_eraseblocks)
 		local ebsize=$(cat /sys/devices/virtual/ubi/$ubidev/eraseblock_size)
-		local avail_size=$(( $availeb * $ebsize ))
+		local avail_size=$((availeb * ebsize))
 		local rootfs_data_size_param="-m"
 		if [ -n "$rootfs_data_max" ] &&
 		   [ "$rootfs_data_max" != "0" ] &&