bnxt_en: Modify ethtool -l|-L to support combined or rx/tx rings.

The driver can support either all combined or all rx/tx rings.  The
default is combined, but the user can now select rx/tx rings.

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