net: bgmac: Mask interrupts during probe

We can have interrupts left enabled form e.g: the bootloader which used
the network device for network boot. Make sure we have those disabled as
early as possible to avoid spurious interrupts.

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