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.

Sponsored by: Chelsio

(cherry picked from commit e07ac3f2fd7336e04178d116033989a6c099fec4)