sfxge(4): add the max number of RSS exclusive contexts

The patch adds enc_rx_scale_max_exclusive_contexts member
to nic_cfg_t structure and sets the corresponding values
for Siena, Huntington and Medford

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18077