gianfar: Don't enable RX Filer if not supported

After commit 15bf176d ("gianfar: Don't enable the Filer w/o the
Parser"), 'TSEC' model controllers (for example as seen on MPC8541E)
always have 8 bytes stripped from the front of received frames.
Only 'eTSEC' gianfar controllers have the RX Filer capability (amongst
other enhancements). Previously this was treated as always enabled
for both 'TSEC' and 'eTSEC' controllers.
In commit 15bf176d

 ("gianfar: Don't enable the Filer w/o the Parser")
a subtle change was made to the setting of 'uses_rxfcb' to effectively
always set it (since 'rx_filer_enable' was always true). This had the
side-effect of always stripping 8 bytes from the front of received frames
on 'TSEC' type controllers.

We now only enable the RX Filer capability on controller types that
support it, thereby avoiding the issue for 'TSEC' type controllers.

Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
Signed-off-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Reviewed-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>