farsync: Fix confusion about DMA address and buffer offset types

Use dma_addr_t for DMA address parameters and u32 for shared memory
offset parameters.

Do not assume that dma_addr_t is the same as unsigned long; it will
not be in PAE configurations.  Truncate DMA addresses to 32 bits when
printing them.  This is OK because the DMA mask for this device is
32-bit (per default).

Also rename the DMA address parameters from 'skb' to 'dma'.

Compile-tested only.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>