mtd: spi-nor: Get rid of nor->page_size

nor->page_size duplicated what nor->params->page_size indicates
for no good reason. page_size is a flash parameter of fixed value
and it is better suited to be found in nor->params->page_size.

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/20211029172633.886453-5-tudor.ambarus@microchip.com