TTY: serial/mpsc, clean up init/remove functions

There is a chain of up to 4 nested ifs in init and remove functions.
Instead, make the code linear and use goto's to handle failures.

Remove unneeded cast from mpsc_release_port by referencing pi->port
directly. And finally, use dev_dbg instead of pr_debug given we have
dev->dev node.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>