iscsi: Mark iSCSI CAM sims as non-pollable.

Previously, iscsi_poll() just panicked. This meant if you got a panic
on a box when using the iSCSI initiator, the attempt to shutdown would
trigger a nested panic and never write out a core. Now, CCB's sent to
iSCSI devices (such as the sychronize-cache request in dashutdown())
just fail with a timeout during a panic shutdown.

Reviewed by: scottl, mav
MFC after: 2 weeks
Sponsored by: Chelsio
Differential Revision: https://reviews.freebsd.org/D28455