ARM: Kirkwood: clk_register_gate_fn: add fn assignment

In commit:
    98d9986c ARM: Kirkwood: Replace clock gating
the kirkwood clock gating has been reworked. A custom variant of
clock gating, that calls a custom function before gating the clock
off, has been introduced. However in clk_register_gate_fn() this
custom function "fn" is never assigned.

This patch adds the missing fn assignment.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@blackshift.org>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Olof Johansson <olof@lixom.net>