mtd: spi-nor: swp: Improve code around spi_nor_check_lock_status_sr()

- bool return value for spi_nor_check_lock_status_sr(), gets rid of
  the return 1,
- introduce temporary variables for better readability.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20210322075131.45093-3-tudor.ambarus@microchip.com