tty: fix kernel-doc for functions in tty.h

tty_kref_get() is already included in Documentation, but is not properly
formatted. Fix this.

tty_get_baud_rate() is neither properly formatted, nor is included. Fix
both.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230919085156.1578-10-jirislaby@kernel.org


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