net: mvpp2: use "dma" instead of "phys" where appropriate

As indicated by Russell King, the mvpp2 driver currently uses a lot
"phys" or "phys_addr" to store what really is a DMA address. This commit
clarifies this by using "dma" or "dma_addr" where appropriate.

This is especially important as we are going to introduce more changes
where the distinction between physical address and DMA address will be
key.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>