mmc: core: Fix clock frequency transitions during invalid states

eMMC and SD card specifications restrict the usage of a class of
commands while commands in other class are in progress. For example,
during erase operations the SD/eMMC spec. allows only CMD35, CMD36,
CMD38. If clock scaling is enabled and decide to scale up the clocks
it may be possible that CMD19/21 tuning commands are sent in between
erase commands, which is illegal as per specification.

Fix such illegal transactions to the card and also make clock scaling
statistics accountable only for read/write commands instead of time
consuming commands, like CMD38 erase, where transactions are independent
of bus frequency.

Change-Id: Iffba175787837e7f95bde8970f19d0f0f9d7d67d
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>