mtd: tests: nandbiterrs: Fix read_page return value

The number of corrected bitflips is not correctly reported by
the test until the bitflip threshold is reached.

read_page() shall return the number of corrected bitflips, but
mtd_read() returns 0 or a negative error, so we can't forward
its return value. In the absence of an error we always have
calculate the number of bitflips ourselves.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>