i2c: Restore i2c_smbus_process_call function

Restore the i2c_smbus_process_call() as one driver (for the
Micronas MAP5401) will need it soon.

[JD: Update documentation accordingly.]

Signed-off-by: Prakash Mortha <pmortha@escient.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>