drivers: adc: stm32: don't fail init if pinctrl is not provided

Commit 47187a9e made the `pinctrl` property
of STM32 ADCs optional, to allow usage of internal channels without wasting
GPIO pins. However, the driver was not adapted to support this new usecase.

(The real bug comes from commit 93956b20,
that transitioned from a custom `stm32_dt_pinctrl_configure` function to
the standard `pinctrl_apply_state`, without accounting for the fact that
the former returns 0 when pinctrl is empty, but the latter returns -ENOENT)

Modify the driver to work even if no `pinctrl` is present on the ADC node.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>