atmel-mci: Don't stop the clock between transfers

Some cards might get upset if we turn off the clock for extended periods
of time. So keep the clock running until the mmc core tells us to turn
it off.

Also, don't reset the controller between each transfer. That was an
attempt to work around earlier bugs, and it never really worked very
well.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>