Set rxr->next_to_refresh correctly in ixgbe_setup_receive_ring().

ixgbe_setup_receive_ring() fully allocates rx buffers. When a
descriptor ring is full, rxr->next_to_refresh should point to
rxr_next_to_check -1. Before this change, rxr->next_to_refresh
is set to 0 and ixgbe_refresh_mbufs() wastefully loops in
ixgbe_refresh_mbufs() because it means the ring is empty.

(msaitoh)