This moves the initialization of the struct scsi_device fields
use_10_for_ms and use_10_for_rw into a central place.  This allows the
host's slave_configure() function to change them (with effect).  This
also collapses two identical sections of code into a single one.

This is needed by usb-storage, so we can resolve some Babble problems,
some device-crashing problems, and remove a great deal of troublesome
mode-sense-translation code.