serial: rda: Use uart_prepare_sysrq_char() to handle sysrq.

serial: rda: Use uart_prepare_sysrq_char() to handle sysrq.

The port lock is a spinlock_t which is becomes a sleeping lock on PREEMPT_RT.
The driver splits the locking function into two parts: local_irq_save() and
uart_port_lock() and this breaks PREEMPT_RT.

Handle sysrq requests sysrq once the port lock is dropped.
Remove the special case in the console write routine an always use the
complete locking function.

Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-unisoc@lists.infradead.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20240301215246.891055-12-bigeasy@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>