drivers: i2c: stm32: Simplify device init

pm_device_runtime_enable() will call i2c_stm32_suspend() if
pm_device_init_suspended() isn't called.
Since the aim is to perform suspension, just need to call
pm_device_runtime_enable().

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>