Improve support of SCSI cards based on the 53c860 and 53c875.
Still no support for Ultra-SCSI and other new features, but the code
should now correctly initialize the clock pre-scaler (based on freqency
measurement results, if necessary).

Fix support of 16 targets for WIDE SCSI.

Disable bus reset in case no progress is made for too long ("ncr dead"
message), which did not work too well with scanners and other slow devices.