tty: n_tty: rename and retype 'retval' in n_tty_ioctl()

The value stored to the current 'retval' is number of characters. It is
both obtained and put to user as unsigned. So make its type unsigned.
And provided it's not a "return value" per se, rename it to 'num'.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230919085156.1578-3-jirislaby@kernel.org


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