mmc: core: export function mmc_do_release_host()

When using mmc_try_claim_host the corresponding release
function is mmc_do_release_host, which then also must
be exported.

Reviewed-by: Jonas Aberg <jonas.aberg@stericsson.com>
Reviewed-by: Sebastian Rasmussen <sebastian.rasmussen@stericsson.com>
Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>