mxser: fix xmit_buf leak in activate when LSR == 0xff

In the Linux kernel, the following vulnerability has been resolved: mxser: fix xmit_buf 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 by properly freeing the buffer in a designated label. We jump there also from the "!info->type" if now too.