x86/tsc: Make __use_tsc __ro_after_init

__use_tsc is only ever enabled in __init tsc_enable_sched_clock(), so mark
it as __ro_after_init.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240313180106.2917308-5-vschneid@redhat.com