mtd: rawnand: ams-delta: Set port direction when needed

In its current shape, the driver sets data port direction before each
byte read/write operation, even during multi-byte transfers.  Improve
performance of the driver by setting the port direction only when
needed.

This optimisation will become particularly important as soon as
planned conversion of the driver to GPIO API for data I/O will be
implemented.

Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>