mtd: m25p80: Make jedec_probe() return proper errno values

spi_write_then_read() may return its own return codes (e.g. -EIO),
so let's propagate the value down to the probe().

Also, remove jedec == 0 check, it isn't needed as nowadays we use
dedicated SPI device IDs for non-JEDEC flashes.

Suggested-by: Barry Song <21cnbao@gmail.com>
Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>