net: fec: fix missing napi_disable call

Commit dc975382

 introduces napi support
but never calls napi_disable. This will generate a kernel oops
(kernel BUG at include/linux/netdevice.h:473!) every time, when
ndo_stop is called followed by ndo_start.
Add the missing napi_diable call.

Signed-off-by: Georg Hofmann <georg@hofmannsweb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>