drivers: i2c: stm32: Don't abuse of pm_device_runtime_get/put

In driver runtime configuration function, calling
calling pm_device_runtime_get/put() will have the effect of performing
pinctrl change from sleep to default and back from default to sleep
which is useless and in turn enables and disables GPIO clocks two times.

Stop this crazyness and purely enable/disable clock, which might be
superfluous in some cases but which remains much more reasonable than
than the previous implementation.

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