CVE-2022-49191 mxser: fix xmit_buf leak in activate when LSR == 0xff

In the Linux kernel, the following vulnerability has been resolved: mxser: fix xmitbuf leak in activate when LSR == 0xff When LSR is 0xff in -activate rather unlike, we return an error. Provided -shutdown is not called when -activate fails, nothing actually frees the buffer in this case. Fix this...