bna: remove TX_E_PRIO_CHANGE event and BNA_TX_F_PRIO_CHANGED flag

TX_E_PRIO_CHANGE event is never sent for bna_tx so it doesn't need to be
handled. After this change bna_tx->flags cannot contain
BNA_TX_F_PRIO_CHANGED flag and it can be also eliminated.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>