mmc: mvsdio: use slot-gpio for card detect 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 card detect
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>