mtd: spi-nor: Constify part specific fixup hooks

Constify 'struct spi_nor_fixups' in order to respect flash_info
structure declaration.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20211106102915.153552-1-tudor.ambarus@microchip.com