linux-user: fix up oversealous nitpicking

Looks like linux-user code was correct, just unreadable: what it wanted
to do with "-=" was really assign a negative number, not decrement.  Fix
up accordingly.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
