mtd-utils: Fix "are we really at EOF" test logic in libubi read_data

The function reads file data into a buffer and then checks if we
actually are at the end-of-file by trying to read one more byte.
For whatever reason, the code uses an int instead of a char. It's
not pretty but works. But again, this is something that every
static analysis tool barks at.

Further more, the error messages are inverted. "We aren't at EOF yet"
is printed on failure and something like "read error %m" is printed
on success.

This patch fixes all of the above.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>