ramips: Fix setting of rx buffer length

ramips: Fix setting of rx buffer length

The length of the DMA rx buffer was always set to 0 because the function
for extracting the length was used to calculate the value for setting it.
Instead the macro has to be split in a get and set function similar to the
TX_DMA_(GET_|)PLEN(0|1) macro.

No problem was noticed on MT7621a before this was changed and thus maybe it
was hidden by different problem which is not yet fixed.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>
SVN-Revision: 47115