DZ failure mishandling

[utzoo!henry]

The Bell DZ driver (and the fixed-up one from Duke as well) can infinite-
loop in the presence of certain kinds of hardware failures.  The scanning
loop in the transmitter-interrupt routine runs forever if stuck bits cause
the TRDY bit to stay on.  We had this happen not long ago.  The infinite
loop is at interrupt priority and hence effectively hangs the system.

A reasonable way to be paranoid about this is to decrement a counter from,
say, 10000, and drop out if it hits zero.  Our driver, in such circumstances,
prints a message and sets the offending DZ's CSR to 0 to try and get rid of
it long enough for an orderly shutdown.