Use IS_ENABLED(CONFIG_DEBUG_FS) to return in case debugfs is not present.
Since this is known at compile time, it allows the compiler to drop any
unused code. Therefore no need to wrap the code with #ifdef.

While at it make adis16400_debugfs_init() void as the return code is
ignored.
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240809-dev-adis-debugfs-improv-v1-3-d3adb6996518@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>