rte_eal_check_module() might return -1, which would have been a
"not false" condition for mod_available. Fix that to only report
vfio being enabled if rte_eal_check_module() returns 1.
Fixes: 221f7c220d6b ("vfio: move global config out of PCI files")
Cc: stable@dpdk.org
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>