mmc: tmio_mmc: Hotplug code regrouping

This patch regroups the code slightly, adds documentation and allows
the rtpm counter of MMC_CAP_NEEDS_POLL devices to reach 0 again.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
[g.liakhovetski@gmx.de: restore pm_runtime_get_noresume()]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>