net: mvpp2: simplify the Tx desc set DMA logic

Two functions were always used to set the DMA addresses in Tx
descriptors, because this address is split into a base+offset in the
descriptors. A mask was used to come up with the base and offset
addresses and two functions were called, mvpp2_txdesc_dma_addr_set() and
mvpp2_txdesc_offset_set().

This patch moves the base+offset calculation logic to
mvpp2_txdesc_dma_addr_set(), and removes mvpp2_txdesc_offset_set() to
simplify things.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>