cxgb3: fix dma mapping regression

Commit 5e68b772
  cxgb3: map entire Rx page, feed map+offset to Rx ring.

introduced a regression on platforms defining DECLARE_PCI_UNMAP_ADDR()
and related macros as no-ops.

Rx descriptors are fed with the a page buffer bus address + page chunk offset.
The page buffer bus address is set and retrieved through
pci_unamp_addr_set(), pci_unmap_addr().
These functions being meaningless on x86 (if CONFIG_DMA_API_DEBUG is not set).
The HW ends up with a bogus bus address.

This patch saves the page buffer bus address for all plaftorms.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>