IIO: ADC: stm32-dfsdm: avoid unused-variable warning

Building with CONFIG_OF disabled produces a compiler warning:

drivers/iio/adc/stm32-dfsdm-core.c: In function 'stm32_dfsdm_probe':
drivers/iio/adc/stm32-dfsdm-core.c:245:22: error: unused variable 'pnode' [-Werror=unused-variable]

This removes the variable and open-codes it in the only place
it gets used to avoid that warning.

Fixes: bed73904 ("IIO: ADC: add stm32 DFSDM core support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>