mmc: tmio-mmc: Remove .get_cd() callback from platform data

All platforms pass the CD GPIO number to the driver in the .cd_gpio
field. The .get_cd() callback isn't used anymore, remove it

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>