mmc: core: use the defined function to check whether card is removable

In linux/mmc/host.h, mmc_card_is_removable() is already defined.
There is no reason that it doesn't use.

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