net: mvneta: move the RX and TX desc macros outside of the structs

The macros used for the various fields of the RX and TX descriptions
are currently declared next to those fields within the structure
definitions of the RX and TX descriptors.

However, in order to support big endian, we'll have to use the "swap
descriptors" features of the hardware, which swaps every byte within
each 64 bits word of the descriptors. This requires a separate
definition of the RX and TX descriptor structures for little and big
endian, as is done in the mv643xx_eth. Those macros can therefore no
longer be defined inside those structures.

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