sfc: Correct efx_rx_buffer::page_offset when EFX_PAGE_IP_ALIGN != 0

RX DMA buffers start at an offset of EFX_PAGE_IP_ALIGN bytes from the
start of a cache line.  This offset obviously needs to be included in
the virtual address, but this was missed in commit b590ace0
('sfc: Fix efx_rx_buf_offset() in the presence of swiotlb') since
EFX_PAGE_IP_ALIGN is equal to 0 on both x86 and powerpc.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>