lib: sbi: Have spinlock checks return bool

spin_lock_check already returned bool in the source file but not in the
header. With some toolchains that causes an error, as it should.

Because it and related functions all essentially return a bool, we can
use this opportunity to change them.

Signed-off-by: Daniel Schaefer <git@danielschaefer.me>
Reviewed-by: Anup Patel <anup.patel@wdc.com>