mmc: mmci: Clarify comments and some code for busy detection

The code dealing with busy detection is somewhat complicated. In a way to
make it a bit clearer, let's try to clarify the comments in the code about
it.

Additionally, move the part for clearing the so called busy start IRQ, to
the place where the IRQ is actually delivered. Ideally, this should make
the code a bit more robust.

Finally, to improve understanding of the code and the sequence of the busy
detection, move the corresponding code around a bit in mmci_cmd_irq().

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Jean Nicolas GRAUX <jean-nicolas.graux@st.com>
Reviewed-by: Jean Nicolas GRAUX <jean-nicolas.graux@st.com>