opp: rust: mark OPP methods as inline

opp: rust: mark OPP methods as inline

When building the kernel using llvm-19.1.7-rust-1.85.0-x86_64, the
following symbols are generated:

$ nm vmlinux | grep ' _R'.*OPP | rustfilt
ffffffff81801560 T <kernel::opp::OPP>::freq
ffffffff81801540 T <kernel::opp::OPP as kernel::sync::aref::AlwaysRefCounted>::dec_ref
ffffffff81801520 T <kernel::opp::OPP as kernel::sync::aref::AlwaysRefCounted>::inc_ref

However, these Rust symbols are trivial wrappers around the
`dev_pm_opp_get`, `dev_pm_opp_put` and `dev_pm_opp_get_freq_indexed`
functions. It doesn't make sense to go through a trivial wrapper for
these functions.

After applying this patch, the above command will produce no output.

Link: https://github.com/Rust-for-Linux/linux/issues/1145
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Nicolás Antinori <nico.antinori.7@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>