ubiformat: Fix formatting devices with unreadable areas
When parts of a Nand device are not readable due to excessive bit flips
we should not bail out with an error as this means we can never repair
the device by flashing a fresh image using ubiformat. Instead, treat a
failed read as a corrupt block and ignore the read failure.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>