mtd: rawnand: Clarify conditions to enable continuous reads

The current logic is probably fine but is a bit convoluted. Plus, we
don't want partial pages to be part of the sequential operation just in
case the core would optimize the page read with a subpage read (which
would break the sequence). This may happen on the first and last page
only, so if the start offset or the end offset is not aligned with a
page boundary, better avoid them to prevent any risk.

Cc: stable@vger.kernel.org
Fixes: 003fe4b9

 ("mtd: rawnand: Support for sequential cache reads")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/linux-mtd/20231215123208.516590-5-miquel.raynal@bootlin.com