mtd: spi-nor-core: Use dev_err for reporting erase/write failures

Use dev_err() when reporting reason for erase/write failures so that
users can be made aware of the reason for failure.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>