sfc: Do not free an empty page_ring

When the page_ring is not used page_ptr_mask is 0.
Do not dereference page_ring[0] in this case.

Fixes: 2768935a

 ("sfc: reuse pages to avoid DMA mapping/unmapping costs")
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>