gianfar: Fix on-the-fly vlan and mtu updates

The RCTRL and TCTRL registers should not be changed
on-the-fly, while the controller is running, otherwise
unexpected behaviour occurs.  But that's exactly what
gfar_vlan_mode() does, updating the VLAN acceleration
bits inside RCTRL/TCTRL.  The attempt to lock these
operations doesn't help, but only adds to the confusion.
There's also a dependency for Rx FCB insertion (activating
/de-activating the TOE offload block on Rx) which might
change the required rx buffer size.  This makes matters
worse as gfar_vlan_mode() ends up calling gfar_change_mtu(),
though the MTU size remains the same.  Note that there are
other situations that may affect the required rx buffer size,
like changing RXCSUM or rx hw timestamping, but errorneously
the rx buffer size is not recomputed/ updated in the process.

To fix this, do the vlan updates properly inside the MAC
reset and reconfiguration procedure, which takes care of
the rx buffer size dependecy and the rx TOE block (PRSDEP)
activation/deactivation as well (in the correct order).
As a consequence, MTU/ rx buff size updates are done now
by the same MAC reset and reconfig procedure, so that out
of context updates to MAXFRM, MRBLR, and MACCFG inside
change_mtu() are no longer needed.  The rx buffer size
dependecy to Rx FCB is now handled for the other cases too
(RXCSUM and rx hw timestamping).

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>