net: mvmdio: do not clk_disable_unprepare() NULL clock

There is no need to clk_disable_unprepare(dev->clk)
before it was initialized.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>