mmc: sdhci: clean up interrupt handling

sdhci interrupt handling is a mess; there is a lot of code doing very
similar things.  Let's clean this up a bit:

1. set's clear down cmd, data and bus power interrupts in one go - we're
   always going to handle these.
2. use a do { } while () loop for looping while there are pending
   interrupts.
3. group clearing of bits in intmask into one place.

This results in the code becoming simpler and easier to read.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>