Adds a new function pointer to struct if_txrx in order to allow
drivers to set their own function that will determine which queue
a packet should be sent on.
(This motivation behind this is to allow the driver to examine the
UP in the VLAN tag and determine which queue to TX on based on
that, in support of HW TX traffic shaping.)
Signed-off-by: Eric Joyner <erj@FreeBSD.org>