sata_mv: safer logic for limit_warnings

There is a miniscule chance that two separate host controllers
might be in sata_mv at the same time and manage to decrement
the static limit_warnings variable below zero.

Fix the comparison to deal with it.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>