mtdparts: Fix uninitialized scalar usage

When reworking this code to fix other issues found by Coverity, I forgot
to ensure tmp_ep was always cleared before use.

Reported-by: Coverity (CID: 166612)
Fixes: bc028345 ("mtdparts: Fix final outstanding issue reported by Coverity")
Signed-off-by: Tom Rini <trini@konsulko.com>