tty: moxa: fix TIOCSSERIAL permission check

tty: moxa: fix TIOCSSERIAL permission check

BugLink: https://bugs.launchpad.net/bugs/1929455
commit dc8c8437658667be9b11ec25c4b5482ed2becdaa upstream.

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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>