eth: fbnic: add coverage for RXB stats

eth: fbnic: add coverage for RXB stats

This patch provides coverage to the RXB (RX Buffer) stats. RXB stats
are divided into 3 sections: RXB enqueue, RXB FIFO, and RXB dequeue
stats.

The RXB enqueue/dequeue stats are indexed from 0-3 and cater for the
input/output counters whereas, the RXB fifo stats are indexed from 0-7.

The RXB also supports pause frame stats counters which we are leaving
for a later patch.

ethtool -S eth0 | grep rxb
     rxb_integrity_err0: 0
     rxb_mac_err0: 0
     rxb_parser_err0: 0
     rxb_frm_err0: 0
     rxb_drbo0_frames: 1433543
     rxb_drbo0_bytes: 775949081
     ---
     ---
     rxb_intf3_frames: 1195711
     rxb_intf3_bytes: 739650210
     rxb_pbuf3_frames: 1195711
     rxb_pbuf3_bytes: 765948092

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250410070859.4160768-4-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>