tty: serial: samsung: Remove redundant port lock acquisition in rx helpers

In the Linux kernel, the following vulnerability has been resolved:

tty: serial: samsung: Remove redundant port lock acquisition in rx helpers

Sashiko identified a deadlock when the console flow is engaged [1].

When console flow control is enabled (UPF_CONS_FLOW),

s3c24xx_serial_stop_tx() calls s3c24xx_serial_rx_enable() and

s3c24xx_serial_start_tx() calls s3c24xx_serial_rx_disable().

The serial core framework invokes the .stop_tx() and .start_tx()

callbacks with the port->lock spinlock already held. Furthermore, all

internal driver paths that invoke stop_tx (such as the DMA TX

completion handler s3c24xx_serial_tx_dma_complete() or the PIO TX IRQ

handler s3c24xx_serial_tx_irq()) also acquire port->lock prior to

calling it. (Note that s3c24xx_serial_start_tx() is only invoked by the

serial core).

However, s3c24xx_serial_rx_enable() and s3c24xx_serial_rx_disable()

unconditionally attempt to acquire port->lock again using

uart_port_lock_irqsave(). Since spinlocks are not recursive, this

causes a deadlock on the same CPU when console flow control is engaged.

Remove the redundant lock acquisition from both rx helper functions.