sfxge(4): honour packed stream RSS restriction

Packed stream firmware variant on EF10 adapters has a
number of properties which must be taken into account:

• Only one exclusive RSS context is available per port.
• Only IP addresses can contribute to the hash value.

Huntington and Medford have one more limitation which
is important for the drivers capable of packed stream:

• Hash algorithm is non-standard (i.e. non-Toeplitz). This implies XORing together source + destination IP addresses (or last four bytes in the case of IPv6) and using the result as the input to a Toeplitz hash.

This patch provides a number of improvements in order
to treat the mentioned limitations in the common code.

If the firmware variant is packed stream, the list of
supported hash tuples will include less variants, and
the maximum number of RSS contexts will be set to one.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18232