mtd: spi-nor: Rename label as it is no longer generic

Rename 'sst_write_err' label to 'out' as it is no longer generic for
all the errors in the sst_write() method, and may introduce confusion.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>