i2c: Last message in transfer implies STOP

The last message in an i2c_transfer msg array implies a STOP. This
documents the expected behavior of the i2c_transfer command a bit more
in that every i2c_transfer() call is expected to leave the bus in a
useful state for the next subsequent call to i2c_transfer().

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>