Avoid enabling MSI-X if MSI-X is disabled globally

It was reported on the community call that with
hw.pci.enable_msix=0, iflib would enable MSI-X on the device and attempt
to use it, which caused issues. Test the sysctl explicitly and do not
enable MSI-X if it's disabled globally.

Reviewed by: sbruno
Approved by: sbruno (mentor)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12805