eth: fbnic: update fbnic_poll return value

In cases where the work done is less than the budget, `fbnic_poll` is
returning 0. This affects the tracing of `napi_poll`. Following is a
snippet of before and after result from `napi_poll` tracepoint. Instead,
returning the work done improves the manual tracing.

Before:
@[10]: 1
...
@[64]: 208175
@[0]: 2128008

After:
@[56]: 86
@[48]: 222
...
@[5]: 1885756
@[6]: 1933841

Signed-off-by: Mohsin Bashir <[email protected]>
Reviewed-by: Jakub Kicinski <[email protected]>
Reviewed-by: Kalesh AP <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: Mohsin Bashir

drivers/net/ethernet/meta/fbnic/fbnic_txrx.c

@@ -1033,7 +1033,7 @@ static int fbnic_poll(struct napi_struct *napi, int budget)
 	if (likely(napi_complete_done(napi, work_done)))
 		fbnic_nv_irq_rearm(nv);
 
-	return 0;
+	return work_done;
 }
 
 irqreturn_t fbnic_msix_clean_rings(int __always_unused irq, void *data)