hw/i2c: Add a NULL check for i2c slave init callbacks

Add a NULL check for i2c slave init callbacks, so that we no longer
need to implement empty init functions.

Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
Message-id: 20161202054617.6749-4-alastair@au1.ibm.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: squashed in later tweak from Alistair to if() phrasing]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>