bnxt_en: Add ethtool -S priority counters.

Display the CoS counters as additional priority counters by looking up
the priority to CoS queue mapping.  If the TX extended port statistics
block size returned by firmware is big enough to cover the CoS counters,
then we will display the new priority counters.  We call firmware to get
the up-to-date pri2cos mapping to convert the CoS counters to
priority counters.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>