alc(4): disable MSI-X by default on Killer cards

Several users with alc(4)-based "Killer" Ethernet cards have reported
issues with this driver not passing traffic, which are solved by
disabling MSI-X using the provided tunable.

To work around this issue, disable MSI-X by default on this card.

This is done by having msix_disable default to 2, which means
"auto-detect". The user can still override this to either 0 or 1 as
desired.

Since these are slow (1Gbps) Ethernet ICs used in low-end systems, it's
unlikely this will cause any practical performance issues; on the other
hand, the card not working by default likely causes issues for many new
FreeBSD users who find their network port doesn't work and have no idea
why.

PR: 230807
MFC after: 1 week

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1185

(cherry picked from commit 05a95d19cb248203acdd4e069d3eedfe597c3b49)