iio: frequency: adf4377: Fully initialize clk_init_data and clk_parent_data

In the Linux kernel, the following vulnerability has been resolved:

iio: frequency: adf4377: Fully initialize clk_init_data and clk_parent_data

The clk_init_data structure contains several mutually-exclusive members
for different methods to specify the possible parents of a clock,
prompting drivers to initialize only the members they need. However,
not initializing all members may cause subtle issues, which are only
exposed when CONFIG_INIT_STACK_ALL_PATTERN or CONFIG_INIT_STACK_NONE is
enabled.

adf4377_clk_register() fills in init.parent_data, and assumes that
init.parent_names is NULL. However, the latter is uninitialized, and
thus may cause a crash.

Similarly, adf4377_clk_register() fills in only parent_data.fw_name,
leaving other members of the clk_parent_data structure uninitialized.

Make sure all members are fully initialized, to fix such bugs, and to
avoid future breakage when converting drivers to a different method for
specifying the parents.