fix compiler warning in fixed.c

Correct the following compiler warning (and warnings resulting from
the correction):
warning: 'fixed_mdio_register_device' defined but not used

Signed-off-by: Denver Gingerich <denver@ossguy.com>
Cc: Vitaly Bordug <vbordug@ru.mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>