mmc: sh-mmcif: simplify IRQ processing

The classical way to process IRQs is read out the status, ack all triggered
IRQs, possibly mask them, then process them. Follow this simple procesure
instead of the current complex custom algorithm.

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