Fix comment in include/linux/mmc/host.h

In include/linux/mmc/host.h, it is mentionned that the callback to
know if a card is present or not is get_ro(). But it's get_cd().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>