sfxge(4): fix GET_RXDP_CONFIG usage for multi-PF on Medford

On Medford, using MC_CMD_GET_RXDP_CONFIG to query the RX end
padding setting is in the ADMIN group, and so fails for
unprivileged functions. In that case, assume the largest size
supported by Medford hardware (256bytes) to prevent overrun.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8933