mmc: core: Factor out code related to polling in __mmc_switch()

In yet another step of cleaning up __mmc_switch(), let's factor out the
code that deals with card busy polling.

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>