net: gianfar: remove the unneeded check of disabled device

Since commit cd1e6504

 ("of/device: Don't register disabled
devices"), the disabled device will not be registered at all. So we
don't need to do the check again in the platform device driver.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>