Some I2C bus implementations need to synchronize with external
entities, such as system firmware, which might also be programming the
same I2C bus.

In order to facilitate this add ->xfer_begin() and ->xfer_end() hooks
which are invoked around pcf_xfer().

[JD: Make these hooks optional.]
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NJean Delvare <khali@linux-fr.org>