eth: bnxt: fix counting packets discarded due to OOM and netpoll

I added OOM and netpoll discard counters, naively assuming that
the cpr pointer is pointing to a common completion ring.
Turns out that is usually *a* completion ring but not *the*
completion ring which bnapi->cp_ring points to. bnapi->cp_ring
is where the stats are read from, so we end up reporting 0
thru ethtool -S and qstat even though the drop events have happened.
Make 100% sure we're recording statistics in the correct structure.

Fixes: 907fd4a2

 ("bnxt: count discards due to memory allocation errors")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20240424002148.3937059-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>