CVE-2026-64528: 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 (UPFCONSFLOW), s3c24xxserialstoptx() calls s3c24xxserialrxenable() and s3c24xxserialstarttx() calls s3c24xxserialrxdisable().

The serial core framework invokes the .stoptx() and .starttx() callbacks with the port->lock spinlock already held. Furthermore, all internal driver paths that invoke stoptx (such as the DMA TX completion handler s3c24xxserialtxdmacomplete() or the PIO TX IRQ handler s3c24xxserialtxirq()) also acquire port->lock prior to calling it. (Note that s3c24xxserialstarttx() is only invoked by the serial core).

However, s3c24xxserialrxenable() and s3c24xxserialrxdisable() unconditionally attempt to acquire port->lock again using uartportlockirqsave(). 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.