mmc: mvsdio: use slot-gpio infrastructure for write protect gpio

The MMC core subsystem provides in drivers/mmc/core/slot-gpio.c a nice
set of helper functions to simplify the management of the write
protect GPIO in MMC host drivers. This patch migrates the mvsdio
driver to using those helpers, which will make the ->probe() code
simpler, and therefore ease the process of adding a Device Tree
binding for this driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Stefan Peter <s.peter@mpl.ch>
Tested-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Chris Ball <cjb@laptop.org>