ice(4): Fix build error when ALTQ is enabled

The previous commit (56429daea2) that updated the driver included a
bug where a variable was undefined when ALTQ was enabled; this fixes
that issue to declare the missing variable. This wasn't caught before
because we don't use ALTQ, and so it fell on the Jenkins CI's LINT
builds to catch it.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reported by: Jenkins CI
MFC after: 3 days
MFC with: 56429daea2
Sponsored by: Intel Corporation