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 <mohsin.bashr@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Link: https://patch.msgid.link/20250104015316.3192946-1-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>