mtd: fix md5sum error checking

In mtd_verify(), the return value of md5sum() has been
interpreted as error if nonzero, while the function
returns number of processed bytes, which caused
mtd_verify() to always fail.

This patch fixes error checking to interpret only
negative values as errors.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44605 3c298f89-4303-0410-b956-a3cf2f4a3e73