net: dsa: mv88e6xxx: better IEEE Prio Mapping Table description

Kill the remaining shift macro in favor of calculating at compile time
its value from the more descriptive mask, which gives us a better
representation of the register layout.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>