mmc: tmio_mmc: merge the private header into the driver

drivers/mmc/host/tmio_mmc.h is only used by drivers/mmc/host/tmio_mmc.c,
this needlessly complicates source-code handling.

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