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.

Reviewed by: scottl, mav (earlier version)
Reviewed by: imp
MFC after: 2 weeks
Sponsored by: Chelsio
Differential Revision: https://reviews.freebsd.org/D28453