mtd: spi-nor: sysfs: hide the flash name if not set

The flash name is not reliable as we saw flash ID collisions.
Hide the flash name if not set.

Signed-off-by: JaimeLiao <jaimeliao@mxic.com.tw>
Reviewed-by: Michael Walle <michael@walle.cc>
[ta: update commit subject and description and the sysfs description]
Link: https://lore.kernel.org/r/20231215082138.16063-4-tudor.ambarus@linaro.org


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>