iflib: Support to padding Ethernet frames to a min size

Some bnxt devices do not correctly send frames smaller than
52 bytes (without CRC), so add a quirk that will pad frames to an
arbitrary size before passing off to the encap routine.

Reported by: Bhargava Chenna Marreddy <bhargava.marreddy@broadcom.com>
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D13269