tty: pcn_uart: fix memory leak with using debugfs_lookup()

In the Linux kernel, the following vulnerability has been resolved:

tty: pcn_uart: fix memory leak with using debugfs_lookup()

When calling debugfs_lookup() the result must have dput() called on it, otherwise the memory will leak over time. To make things simpler, just call debugfs_lookup_and_remove() instead which handles all of the logic at once.