In most contexts '-1' doesn't really mean much to the casual observer.
In almost all cases, it's better to use a human readable define.  In
this case PLATFORM_DEVID_* defines have already been provided for this
purpose.

While we're here, let's be specific about the 'MFD devices' which
failed.  It will help when trying to distinguish which of the 2 sets
of sub-devices we actually failed to register.
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>