preempt/dynamic: Fix setup_preempt_mode() return value

__setup() callbacks expect 1 for success and 0 for failure. Correct the
usage here to reflect that.

Fixes: 826bfeb3

 ("preempt/dynamic: Support dynamic preempt with preempt= boot option")
Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211203233203.133581-1-ahalaney@redhat.com