mmc: Unirqify bcm2835_sdhost and fix writes

mmc: Unirqify bcm2835_sdhost and fix writes

The bcm2835 sdhost driver has a problem with "write multiple" commands.
It seems to boil down to the fact that the controller dislikes its FIFO
to get drained at the end of a block when a write multiple blocks command
is in flight.

The easy fix is to simply get rid of all the IRQ driven logic and make
the driver push as much data into the FIFO as it can. That way we never
drain and we never run into the problem.

Reported-by: Jan Leonhardt <jan@cyberdesigner.net>
Signed-off-by: Alexander Graf <agraf@suse.de>