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.