sfxge(4): add function to create packed stream RxQ

Encoding packed stream buffer size in RxQ type is not a future-proof
idea taking into account a new RxQ types with extra parameters.
To be consistent make packet stream buffer size a separate parameter.
In order to avoid blowing of the default RxQ create function prototype
add a dedicated function to create packed stream RxQ without not
applicable paramters.

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18132