Fix incorrect RX BD producer updates. The producer index was
already updated after allocating mbuf so driver had to use the last
index instead of using next producer index. This should fix driver
hang which may happen under high network load.

Reported by: Igor Sysoev <is <> rambler-co dot ru>, Vlad Galu <dudu <> dudu dot ro>
Tested by: Igor Sysoev <is <> rambler-co dot ru>, Vlad Galu <dudu <> dudu dot ro>
MFC after: 10 days