mmc: sh_mobile_sdhi: remove unused .pdata field

The struct sh_mobile_sdhi_info::pdata field was only used for platform-
based card detection and isn't used anymore since the migration to GPIO-
based MMC slot functions. Remove it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>