GSD-2022-1001822 mxser: fix xmit_buf leak in activate when LSR == 0xff

mxser: fix xmitbuf leak in activate when LSR == 0xff This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.15.33 by commit...