bnxt_en: Refactor ethtool ring statistics logic.

The current code assumes that the per ring statistics counters are
fixed.  In newer chips that support a newer version of TPA, the
TPA counters are also changed.  Refactor the code by defining these
counter names in arrays so that it is easy to add a new array for
a new set of counters supported by the newer chips.

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