mmc: core: fix typo in mmc_stop_host

mmc_stop_host() should be calling mmc_unregister_extcon() to deregister
from the extcon framework but instead it was calling mmc_register_extcon().
Fixed this typo.

Change-Id: I6a235b9b34f01b29aeebf344cb2837f927b47d9c
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>