The serial tty interface also supports additional input signals, that
can also be evaluated within this trigger. This change is adding the
following additional input sources, which could be controlled
via the '/sys/class/<leds>/' sysfs interface.

Explanation:
DCE = Data Communication Equipment (Modem)
DTE = Data Terminal Equipment (Computer)

- cts:
  DCE is ready to accept data from the DTE (CTS = Clear To Send). If
  the line state is detected, the LED is switched on.
  If set to 0 (default), the LED will not evaluate CTS.
  If set to 1, the LED will evaluate CTS.

- dsr:
  DCE is ready to receive and send data (DSR = Data Set Ready). If the
  line state is detected, the LED is switched on.
  If set to 0 (default), the LED will not evaluate DSR.
  If set to 1, the LED will evaluate DSR.

- dcd:
  DTE is receiving a carrier from the DCE (DCD = Data Carrier Detect).
  If the line state is detected, the LED is switched on.
  If set to 0 (default), the LED will not evaluate DCD.
  If set to 1, the LED will evaluate DCD.

- rng:
  DCE has detected an incoming ring signal on the telephone line
  (RNG = Ring Indicator). If the line state is detected, the LED is
  switched on.
  If set to 0 (default), the LED will not evaluate RNG.
  If set to 1, the LED will evaluate RNG.

Also add an invert flag on LED blink, so that the LED blinks in the
correct order.

* If one off the new enabled input signals are evaluatet as 'enabled',
  and data are transmitted, then the LED should first blink 'off' and
  then 'on' (invert).
* If all the new enabled input signals are evaluatet as 'disabled',
  and data are transmitted, then the LED should first blink 'on' and
  then 'off'.
Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Reviewed-by: Maarten Brock <m.brock@vanmierlo.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20231127110311.3583957-5-fe@dev.tdt.deSigned-off-by: Lee Jones <lee@kernel.org>