tty: serial, join uport checks in uart_port_shutdown()

There are two consequent checks of uport != NULL in
uart_port_shutdown(). Join these two under a single block.

De-multiline the comments when shuffling with them anyway.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118071911.12059-5-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>