clk: sunxi-ng: A31: Fix spdif clock register

As the SPDIF was rarely documented on the earlier Allwinner SoCs
it was assumed that it had a similar clock register to the one
described in the H3 User Manual.

However this is not the case and it looks to shares the same setup
as the I2S clock registers.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>