mmc: Protect `mmc_initialize` from initialising mmc multiple times

mmc: Protect `mmc_initialize` from initialising mmc multiple times

`mmc_initialize` might be called multiple times leading to the mmc-controllers
being initialised twice, and initialising the `mmc_devices` list head twice
which may lead to memory leaks.

Signed-off-by: Daniel Kochmański <dkochmanski@turtle-solutions.eu>
CC: Roy Spliet <r.spliet@ultimaker.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
CC: Pantelis Antoniou <panto@antoniou-consulting.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>