mmc: mmci: Move all CMD irq handling to mmci_cmd_irq()

This patch won't change the behavior of how mmci deals with CMD irqs.
By moving code from mmci_irq() to mmci_cmd_irq(), we getter a better
overview of what going on.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>