The unconditional-doorbell fix for Whp/older NICs (always ring the Rx
CQ doorbell after processing Rx interrupts) had also been applied to
P5+ chips, discarding process_nq()'s work-done signal entirely and
losing the doorbell-skip optimization on P5+ when process_nq() finds
no CQ notification to act on.
Make process_nq() return whether it saw a CQ notification, and only
skip the Rx CQ doorbell on P5+ when it didn't; non-P5+ chips continue
to always ring it, since process_nq() isn't used on that path.