mtd: spinand: Gather all the bus interface steps in one single function

mtd: spinand: Gather all the bus interface steps in one single function

Writing the quad enable bit in one helper and doing the chip
configuration in another does not make much sense from a bus interface
setup point of view.

Instead, let's create a broader helper which is going to be in charge of
all the bus configuration steps at once. This will specifically allow to
transition to octal DDR mode, and even fallback to quad (if suppoorted)
or single mode otherwise.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>