[MTD] mtd_dataflash: Incorrect compare-after-write check

After writing to a Dataflash page, the built-in compare operation is
used to check that the page was successfully written.  A logic bug in
checking the results of the comparison currently causes the compare to
never fail.

This bug was originally in the legacy at91_dataflash.c driver.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>