[POWERPC] mpc5200: Fix FEC error handling on FIFO errors

The error handling for the mpc5200 fec interrupt is broken. The intended
behaviour is like this:

* If one of FEC_IEVENT_RFIFO_ERROR and FEC_IEVENT_XFIFO_ERROR happens,
  the datasheet says (MPC5200B User's Guide R1.2, p. 14-13): "When this
  occurs, software must ensure both the FIFO Controller and BestComm are
  soft-reset".

* On any other error (non-TFINT) interrupt, just issue a debug message.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>