i2c: dw: re-organize transfer related code

() Re-organize the steps involved in data transfer, according to
   the datasheet's flowchart.
() Extract the common code for transfer initialization and put
   it into its own function.
() i2c_write() and i2c_polling_write() are now using the common
   data sending function.

Change-Id: Ieb90253ee10ddceb3b5d05b258e7fc6253d18729
Signed-off-by: Daniel Leung <daniel.leung@intel.com>