MFC r234540:

MFC r234540:

Fix the following clang warning in dpt(4):

  sys/dev/dpt/dpt_scsi.c:612:18: error: implicit truncation from 'int' to bitfield changes value from -2 to 2 [-Werror,-Wconstant-conversion]
    dpt->cache_type = DPT_CACHE_WRITEBACK;
                    ^ ~~~~~~~~~~~~~~~~~~~

by defining DPT_CACHE_WRITEBACK as 2, since dpt_softc::cache_type is an
unsigned bitfield.  No binary change.

MFC after:	1 week

git-svn-id: svn://svn.freebsd.org/base/stable/9@234755 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f