firmware: clean up filesystem load exit path

This makes the error and success paths more readable while trying to
load firmware from the filesystem.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Cc: David Howells <dhowells@redhat.com>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>