Building with clang and and W=1 leads to warning about unused
set_test_mode_src_osc_freq_target_low_bits() and
set_test_mode_src_osc_freq_target_hi_bits(). Fix by annotating them with
__maybe_unused.
See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").