sdhci: fix led naming

Fix the led device naming for the sdhci driver.

The led class documentation defines the led name to have the
form "devicename:colour:function" while not applicable sections
should be left blank.

To comply with the documentation the led device name is changed
from "mmc*" to "mmc*::".

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>