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 <[email protected]>
CC: Roy Spliet <[email protected]>
Cc: Ian Campbell <[email protected]>
CC: Pantelis Antoniou <[email protected]>
Acked-by: Hans de Goede <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>