drivers: serial: stm32: store IRQ config function in a single place

When CONFIG_PM=y, the IRQ function also needs to be stored, something
the "generic" uart_device_config cannot support. This uses the config
irq_config_func field to store the function in all situations, thus
removing unnecessary logic and finally dropping uart_device_config.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>