net: bcmgenet: disable clock before register_netdev

As soon as register_netdev() is called, the network device notifiers are
running which means that other parts of the kernel, or user-space
programs can call the network device ndo_open() callback and use the
interface.

Disable the Ethernet device clock before we register the network device
such that we do not create the following situation:

CPU0				CPU1
register_netdev()
				bcmgenet_open()
				clk_prepare_enable()
clk_disable_unprepare()

and leave the hardware block gated off, while we think it should be
gated on.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>