gianfar: Remove sysfs stubs for FIFOCFG and stashing

Removing the sysfs stubs for the Tx FIFOCFG and ATTRELI
(stashing) config registers, as these registers may only
be configured after a MAC reset, with the controller stopped
(i.e. during hw init, at probe() time).  The current sysfs
stubs allow on-the-fly updates of these registers (the locking
measures are useless and only add unecessary code).

Changing these registers is discouraged. Only the default values
will be used instead.

Moreover, the stashing (ATTRELI) configuration options were
effectively disabled (didn't get to the hw anyway if changed)
because the stashing device_flags (HAS_BD_STASHING|HAS_BUF_STASHING)
were "accidentally" cleared during probe().

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