bnxt_en: Add bnxt_get_num_stats() to centrally get the number of ethtool stats.

Instead of duplicating the logic multiple times.  Also, it is unnecessary
to zero the buffer in .get_ethtool_stats() because it is already zeroed
by the caller.

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