staging: fwserial: fix TIOCSSERIAL permission check

Changing the port close-delay parameter is a privileged operation so
make sure to return -EPERM if a regular user tries to change it.

Fixes: 7355ba34 ("staging: fwserial: Add TTY-over-Firewire serial driver")
Cc: stable@vger.kernel.org      # 3.8
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407102334.32361-3-johan@kernel.org


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