bnxt_en: Split HW ring statistics strings into RX and TX parts.

This will allow the RX and TX ring statistics to be separated if needed.
In the next patch, we'll be able to only display RX or TX statistcis if
the channel is RX only or TX only.

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