Allow iflib NIC drivers to sleep rather than busy wait

Since the move to SMP NIC driver locking has had to go through serious
contortions using mtx around long running hardware operations. This moves
iflib past that.

Individual drivers may now sleep when appropriate.

Submitted by: Matthew Macy <mmacy@mattmacy.io>
Reviewed by: shurd
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14983