mmc: block: fix boot partition switch error path

In the case of a switch error, do not update partition config as though
the switch succeeded, and ensure blk_end_request is called on the
failed request.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>