mfd: max77686: Don't define dummy function if OF isn't enabled

When the CONFIG_OF option was not enabled, a dummy function
max77686_i2c_parse_dt_pdata() was defined since this is called
unconditionally on probe(). Just always define the real function
and conditionally call it if CONFIG_OF is enabled instead.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>