CVE-2026-23472 serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN

In the Linux kernel, the following vulnerability has been resolved: serial: core: fix infinite loop in handletx for PORTUNKNOWN uartwriteroom and uartwrite behave inconsistently when xmitbuf is NULL which happens for PORTUNKNOWN ports that were never properly initialized: - uartwriteroom returns...