base-files: fix ipcalc bound calculation for /31 prefix

A small regress from the translation to shell.
Fixes #12921, originally fixed in #12925 (Github).

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>
Link: https://github.com/openwrt/openwrt/pull/15430
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

package/base-files/files/bin/ipcalc.sh

@@ -111,6 +111,8 @@ start=$((network | (start & hostmask)))
 
 if [ "$prefix" -le 30 ]; then
     upper=$(((network | hostmask) - 1))
+elif [ "$prefix" -eq 31 ]; then
+    upper=$((network | hostmask))
 else
     upper="$network"
 fi