iio: adc: ade9000: fix NULL pointer dereference in clkout registration

In the Linux kernel, the following vulnerability has been resolved: iio: adc: ade9000: fix NULL pointer dereference in clkout registration ade9000_setup_clkout() passes NULL as the register address when registering a divider clock. During clock registration, the common clock framework calls clk_divider_recalc_rate(), which dereferences the address through readl(). As a result, probing an ADE9000 configured as a clock provider with an external input clock crashes. CLKOUT passes CLKIN through without changing its rate. Register it as a 1:1 fixed-factor clock, which does not require register access. This change does not affect the configuration using the internal clock, for which the driver does not register a clock provider.