net: mvpp2: add and use accessors for TX/RX descriptors

The PPv2.2 IP has a different TX and RX descriptor layout compared to
PPv2.1. In order to prepare for the introduction of PPv2.2 support in
mvpp2, this commit adds accessors for the different fields of the TX
and RX descriptors, and changes the code to use them.

For now, the mvpp2_port argument passed to the accessors is not used,
but it will be used in follow-up to update the descriptor according to
the version of the IP being used.

Apart from the mechanical changes to use the newly introduced
accessors, a few other changes, needed to use the accessors, are made:

 - The mvpp2_txq_inc_put() function now takes a mvpp2_port as first
   argument, as it is needed to use the accessors.

 - Similarly, the mvpp2_bm_cookie_build() gains a mvpp2_port first
   argument, for the same reason.

 - In mvpp2_rx_error(), instead of accessing the RX descriptor in each
   case of the switch, we introduce a local variable to store the
   packet size.

 - In mvpp2_tx_frag_process() and mvpp2_tx() instead of accessing the
   packet size from the TX descriptor, we use the actual value
   available in the function, which is used to set the TX descriptor
   packet size a few lines before.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>