soc: stm32f1x: fix typo/debug in stop/standby modes

Check for STM32F1 series was done with "#ifdef SOC_SERIES_STM32F1X"
but this symbol comes from Kconfig.

Use the correct "CONFIG_SOC_SERIES_STM32F1X" instead.

Signed-off-by: Oleh Kravchenko <oleg@kaa.org.ua>