Perform I2C combined transactions when possible

Perform I2C combined transactions whenever possible, within the
restrictions of the Broadcomm Serial Controller.
Disable DONE interrupt during TA poll

Prevent interrupt from being triggered if poll is missed and transfer
starts and finishes.
Clear FIFO before starting transfer

Fix driver hang problem. If previous transfer timed out, the next
transfer could deadlock the interrupt handler due to extra bytes present
in FIFO.
Merge pull request #318 from cbeytas/rpi-3.6.y

Perform I2C combined transactions when possible