mmc: core: Don't use extern declarations of public mmc functions

Using extern when declaring functions in the public header, core.h, is
redundant. Let's just remove the use of it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>