net: ethernet: ti: am65-cpsw: update pri_thread_map as per IEEE802.1Q-2014

IEEE802.1Q-2014 supersedes IEEE802.1D-2004. Now Priority Code Point (PCP)
2 is no longer at a lower priority than PCP 0. PCP 1 (Background) is still
at a lower priority than PCP 0 (Best Effort).

Reference:
IEEE802.1Q-2014, Standard for Local and metropolitan area networks
  Table I-2 - Traffic type acronyms
  Table I-3 - Defining traffic types

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>