gianfar: Don't enable the Filer w/o the Parser

Under one unusual circumstance it's possible to wrongly set
FILREN without enabling PRSDEP as well in the RCTRL register,
against the hardware specifications.  With the default config
this does not happen because the default Rx offloads (Rx csum
and Rx VLAN) properly enable PRSDEP.  But if anyone disables
all these offloads (via ethtool), we get a wrong configuration
were the Rx flow classification and hashing, and other Filer
based features (e.g. wake-on-filer interrupt) won't work.
This patch fixes the issue.
Also, account for Rx FCB insertion which happens every time
PRSDEP is set.

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