Don't try reading the SXP_PINS_DIFF on the 10160 and 12160 SCSI
controllers. Reading this register, for which there are indications
that it doesn't really exist, returns 0 on at least some 12160
and doing so on Sun Fire V880 causes a data access error exception.

Reported and tested by: Beat Gaetzi
Approved by: mjacob
Obtained from: OpenBSD (modulo setting isp_lvdmode)