mtd: spi-nor: Skip erase logic when SPI_NOR_NO_ERASE is set

Even if SPI_NOR_NO_ERASE was set, one could still send erase opcodes
to the flash. It is not recommended to send unsupported opcodes to
flashes. Fix the logic and do not set mtd->_erase when SPI_NOR_NO_ERASE
is specified. With this users will not be able to issue erase opcodes to
flashes and instead they will recive an -ENOTSUPP error.

Fixes: b199489d ("mtd: spi-nor: add the framework for SPI NOR")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220228163334.277730-1-tudor.ambarus@microchip.com