mmc: fix sdhci-dove probe/removal

1. Never ever publish a device in the system before it has been setup
   to a usable state.
2. Unregister the device _BEFORE_ taking away any resources it may be
   using.
3. Don't check clks against NULL.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>