i2c: stm32f3: Cleanup how we enable the specific I2C driver

Match change we made to how I2C is enabled for other stm32 platforms:

Right now we allow for the I2C subsystem to be built without any drivers
enabled that utilize it.  When we added support for the new STM32 I2C
driver we forced the I2C driver to be enabled if the I2C subsystem was
enabled.  While this makes a reasonable amount of sense, it breaks
current assumptions for various testcases that we need to cleanup.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>