bnxt_en: show only relevant ethtool stats for a TX or RX ring

Currently, ethtool -S shows all TX/RX ring counters whether the
channel is combined, RX, or TX.  The unused counters will always be
zero.  Improve it by showing only the relevant counters if the channel
is RX or TX.  If the channel is combined, the counters will be shown
exactly the same as before.

[ MChan: Lots of cleanups and simplifications on Rajesh's original
code]

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