UBI: fix missing brace control flow

commit 0e707ae7

 ("UBI: do propagate positive error codes up") seems
to have produced an unintended change in the control flow here.

Completely untested, but it looks obvious.

Caught by Coverity, which didn't like the indentation. CID 1271184.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>