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>

drivers/mmc/mmc.c

@@ -1762,6 +1762,11 @@ static void do_preinit(void)
 
 int mmc_initialize(bd_t *bis)
 {
+	static int initialized = 0;
+	if (initialized)	/* Avoid initializing mmc multiple times */
+		return 0;
+	initialized = 1;
+
 	INIT_LIST_HEAD (&mmc_devices);
 	cur_dev_num = 0;