fm: Don't allow disabling of FM1-DTSEC1

The MDIO controller to talk to external PHYs is on FM1-DTSEC1 so don't
allow disabling.  If we disable it we end up powering the block down in
the SoC and thus can't communicate to any external PHYs.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>