i2c: simplify API and driver_api with generic transfer function

With the introduction of generic transfer function, it is no longer
needed to specify read or write functions explicitly in drivers.
All read/write functions can now thus call the generic transfer
function to achieve the same result.

With this change, the transfer function becomes mandatory, and
should always be available.

Change-Id: Ia6fb98e58b84330a56a5d44ed3df9db42c3a5e88
Signed-off-by: Daniel Leung <daniel.leung@intel.com>