sfxge: [EF10] support RxQ scattering control

If, for example, a VF is configured to use a 1500 byte MTU, but the port
it is attached to is set to 9000 bytes, overlength frames can be received
by the VF. As Huntington scatters by default, these overlength packets
would be scattered across several descriptors, with all except the last
having the CONT bit set.

To avoid this, disable scatter when creating RXQs if the firmware
supports doing so, which all recent versions do. Then we only get
a single descriptor from an overlength frame. This will have the CONT
bit set to indicate it was truncated, so we can discard it.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4354