mtd: mchp48l640: make WEL behaviour configurable

The 48L640 resets the WEL bit (the Write Enable Latch bit in the status
register) to zero on the completion of write operations. In preparation
to support chips behaving differently, introduce .auto_disable_wel
capability, and, if it's missing, explicitly reset the WEL bit after
writes.

Signed-off-by: David Jander <david@protonic.nl>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Jonas Rebmann <jre@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>