mmc: core: Clarify code which deals with polling in __mmc_switch()

The __mmc_switch() deserves a clean-up. In this step, let's move some code
outside of the do-while loop, which deal deals with the card busy polling.

This change simplifies the code in that sense that it becomes easier to follow
what is being executed during card busy polling, but it also gives a better
understanding for when polling isn't done.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>