[MPC512x] Proper handling of larger frames in the FEC driver

When frame larger than local RX buffer is received, it is split and handled
by two buffer descriptors. Prior to this patch the FEC driver discarded
contents of a buffer descriptor without the 'LAST' bit set, so the first
part of the frame was lost in case of larger frames. This fix allows to
safely combine the two pieces into the whole frame.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>