mtd: nand: mxs_nand_spl: don't read useless pages

The patch prevents pages beyond the last from being unnecessarily read.
This occurs when the last page to be read is not the last page of the
last block. Before this change we would have read all the pages up to
the end of the last block.

Suggested-by: Michael Trimarchi <michael@amarulasolutions.com>
Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Link: https://lore.kernel.org/all/20221120095705.3019295-1-dario.binacchi@amarulasolutions.com