bnxt_en: Fix Priority Bytes and Packets counters in ethtool -S.

There is an indexing bug in determining these ethtool priority
counters.  Instead of using the queue ID to index, we need to
normalize by modulo 10 to get the index.  This index is then used
to obtain the proper CoS queue counter.  Rename bp->pri2cos to
bp->pri2cos_idx to make this more clear.

Fixes: e37fed79

 ("bnxt_en: Add ethtool -S priority counters.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>