iflib: Add a new quirk

ENETC NIC found in LS1028A has a bug where clearing TX pidx/cidx
causes the ring to hang after being re-enabled.
Add a new flag, if set iflib will preserve the indices during restart.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: gallatin, erj
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30728