Sleep rather than spin in e1000 when doing long running config operations.

With r333218 it is now possible for drivers to use an sx lock and thus sleep while
waiting on long running operations rather than DELAY().

Reported by: gallatin
Reviewed by: sbruno
Approved by: sbruno
MFC after: 1 month
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14984