bnxt: count discards due to memory allocation errors

Count packets dropped due to buffer or skb allocation errors.
Report as part of rx_dropped.

v2: drop the ethtool -S entry [Vladimir]

Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>