mtd: spi-nor: core: Use auto-detection only once

In case spi_nor_match_name() returned NULL, the auto detection was
issued twice. There's no reason to try to detect the same chip twice,
do the auto detection only once.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220420103427.47867-4-tudor.ambarus@microchip.com