Revert "serial: stm32: Merge hard IRQ and threaded IRQ handling into single IRQ handler"

This reverts commit f24771b6

 as it is
reported to break the build.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/202301200130.ttBiTzfO-lkp@intel.com
Fixes: f24771b6

 ("serial: stm32: Merge hard IRQ and threaded IRQ handling into single IRQ handler")
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Valentin Caron <valentin.caron@foss.st.com> # V3
Cc: Marek Vasut <marex@denx.de>
Cc: Johan Hovold <johan@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>