cam_iosched: use tunable flag and make a bool really a bool

kern.cam.do_dynamic_iosched is really a bool, so change its type to
bool. While I'm here, also use the CTLFLAG_TUN flag instead of a
separate tunable line for it and kern.cam.iosched_alpha_bits.

MFC After: 1 week
Sponsored by: Netflix

(cherry picked from commit a065ccb2804fae834a7c66e29b726ea3dbbceace)