tty: moxa: fix TIOCSSERIAL permission check

Changing the port close delay or type are privileged operations so make
sure to return -EPERM if a regular user tries to change them.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407102334.32361-12-johan@kernel.org


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