i2c/i2c_dw: tiny improvement to execution flow

() Rolls the transfer initialization function into the setup function,
   as it is logical to have just one function (not to mention there
   was duplicate code).
() The setup function returns early if there is any error.

Change-Id: Ie9d3057f2963a0ba5b74ac66e058ff4fee31f099
Signed-off-by: Daniel Leung <daniel.leung@intel.com>