MFC r289726:

MFC r289726:

  Add iicbus_transfer_excl(), a helper routine to do an i2c bus transaction
  while holding exclusive ownership of the bus.  This is the routine most
  slave drivers should use unless they have a need to acquire and hold the
  bus across a series of related operations that involves multiple transfers.

git-svn-id: svn://svn.freebsd.org/base/stable/10@294673 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f