mmc: fix mmc_{un,}register_pm_notifier prototypes

The mmc pm notifiers were recently reworked, but the new
code produces a lot of warnings when CONFIG_PM_SLEEP is disabled:

In file included from ../drivers/mmc/core/sdio_bus.c:27:0:
drivers/mmc/core/core.h:97:13: warning: 'mmc_register_pm_notifier' defined but not used [-Wunused-function]

The obvious solution is to add the 'inline' keyword at the
function definition, as it should be for any function defined
in a header file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0e40be7c20e0 ("mmc: core: Refactor code to register the MMC PM notifier")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>