Several tests in ubiformat test for a positive error code where a
negative error code is returned from the called functions. This
is because the original code used tested against errno which is a
positive value.
One place still tests against errno, but the test should be against
the return value from the last function call. Fix that aswell.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>