sched/core: Use -EINVAL in sched_dynamic_mode()

-1 is -EPERM which is a somewhat odd error to return from
sched_dynamic_write(). No other callers care about which negative
value is used.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20210325004515.531631-2-linux@rasmusvillemoes.dk