bnxt_en: Refactor the software ring counters.

We currently have 3 software ring counters, rx_l4_csum_errors,
rx_buf_errors, and missed_irqs.  The 1st two are RX counters and the
last one is a common counter.  Organize them into 2 structures
bnxt_rx_sw_stats and bnxt_cmn_sw_stats.

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