drivers: spi: stm32: Skip pinctrl setup for subghzspi

Subghzspi instances cannot have any pinctrl configs,
they are blacklisted by the dts binding. This caused an
initialization failure of the spi_ll_stm32 driver for
subghzspi instance because no "default" pinctrl was found.

This commit solves the problem by skipping the pinctrl setup
for subghzpi devices. The use_subghzpi_nss property is used
to identify a subghzspi device, as this is a required boolean
property only available in the subghzspi binding this is a
perfect indicator for such instances.

Signed-off-by: Alexander Mihajlovic <a@abxy.se>