pwm: pca9685: use polling I2C write instead

The polling I2C write function guarantees the write operation
is completed. It is to make sure each write has been commited.

Change-Id: I37cd3b8a65c605837b1fae3ccd1c2b0235c07a37
Signed-off-by: Daniel Leung <daniel.leung@intel.com>