mtd: spi-nor: Fix retlen handling in sst_write()

In case the write of the first byte failed, retlen was incorrectly
incremented to *retlen += actual; on the exit path. retlen should be
incremented when actual data was written to the flash.

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