mtd: spi-nand: Isolate the MTD read logic in a helper

There is currently only a single path for performing page reads as
requested by the MTD layer. Soon there will be two:
- a "regular" page read
- a continuous page read

Let's extract the page read logic in a dedicated helper, so the
introduction of continuous page reads will be as easy as checking whether
continuous reads shall/can be used and calling one helper or the other.

There is not behavioral change intended.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240826101412.20644-4-miquel.raynal@bootlin.com