gianfar: Use memset instead of cacheable_memzero

cacheable_memzero() is completely overkill for the clearing out the FCB
block which is only 8-bytes.  The compiler should easily optimize this
with memset.  Additionally, cacheable_memzero() only exists on ppc32 and
thus breaks builds of gianfar on ppc64.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>