mtd: spi-nor: sysfs: hide manufacturer if it is not set

The manufacturer may be optional when pure SFDP flashes are supported.
Hide the sysfs property if no manufacturer is set.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20220810220654.1297699-3-michael@walle.cc