mmc: ops: export mmc_get_status()

This function retrieves the status of the card with the default
number of retries. Since the block layer wants to use this, and
since the block layer is a loadable kernel module, we need to
export this symbol.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>