Instead of exposing a dedictaed .protect() callback, mimic the
behaviour of corresponding driver in Linux and adjust the value of WP
pin in .write() callback as necessary. This is done in order to
convert this driver to NVMEM subsytem.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>