staging: mt7621-mmc: Fix memory leek in case of error in probe

If the base address is not successfully obtained in the probe
function, then the mmc_host struct is not freed. Adding an exit for
that case fixes the bug.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>