clk: sunxi-ng: a83t: Fix invalid csi-mclk mux offset

Datasheet specified that parent MUX settings are at bits [10:8],
but current implementation specifies incorrect offset at [10:12].
Fix this.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>