gianfar: Revive the driver for eTSEC devices (disable timestamping)

Since commit cc772ab7 ("gianfar: Add
hardware RX timestamping support"), the driver no longer works on
at least MPC8313ERDB and MPC8568EMDS boards (and possibly much more
boards as well).

That's how MPC8313 Reference Manual describes RCTRL_TS_ENABLE bit:

  Timestamp incoming packets as padding bytes. PAL field is set
  to 8 if the PAL field is programmed to less than 8. Must be set
  to zero if TMR_CTRL[TE]=0.

I see that the commit above sets this bit, but it doesn't handle
TMR_CTRL. Manfred probably had this bit set by the firmware for
his boards. But obviously this isn't true for all boards in the
wild.

Also, I recall that Freescale BSPs were explicitly disabling the
timestamping because of a performance drop.

For now, the best way to deal with this is just disable the
timestamping, and later we can discuss proper device tree bindings
and implement enabling this feature via some property.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>