sfxge(4): update RSS API to take RSS context parameter

Update efx_rx_scale_mode_set(), efx_rx_scale_key_set()
and efx_rx_scale_tbl_set().

Submitted by: Mark Spender <mspender at solarflare.com>
Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18080