tg3: Restrict phy ioctl access

If management firmware is present and the device is down, the firmware
will assume control of the phy.  If a phy access were allowed from the
host, it will collide with firmware phy accesses, resulting in
unpredictable behavior.  This patch fixes the problem by disallowing phy
accesses during the problematic condition.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>