cam: Permit non-pollable sims.

Some CAM sim drivers do not support polling (notably iscsi(4)).
Rather than using a no-op poll routine that always times out requests,
permit a SIM to set a NULL poll callback. cam_periph_runccb() will
fail polled requests non-pollable sims immediately as if they had
timed out.

Sponsored by: Chelsio

(cherry picked from commit 447b3557a9cc5f00a301be8404339f21a9a0faa8)
(cherry picked from commit e6405c8c37335a42a187a9d2470025b57347b1b5)