mtd: spi-nor: core: Advance erase after the erase cmd has been completed

addr and len were gratuitously updated even when spi_nor_wait_till_ready()
failed. Wait for the erase cmd to complete and then advance the erase.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210306095002.22983-2-tudor.ambarus@microchip.com