cam: Don't permit crashdumps on non-pollable devices.

If a disk's SIM doesn't support polling, then it can't be used to
store crashdumps. Leave d_dump NULL in that case so that dumpon(8)
fails gracefully rather than having dumps fail at crash time.

Reviewed by: scottl, mav, imp
MFC after: 2 weeks
Sponsored by: Chelsio
Differential Revision: https://reviews.freebsd.org/D28454