mmc: sdhi: Add write16_hook

Some controllers require waiting for the bus to become idle
before writing to some registers. I have implemented this
by adding a hook to sd_ctrl_write16() and implementing
a hook for SDHI which waits for the bus to become idle.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>