leds: ledtrig-tty: Replace mutex with completion

With this commit, the mutex handling is replaced by the completion
handling. When handling mutex, it must always be ensured that the held
mutex is also released again. This is more error-prone should the number
of code paths increase.

This is a preparatory commit to make the trigger more configurable via
additional sysfs parameters. With this change, the worker always runs and
is no longer stopped if no ttyname is set.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20231127110311.3583957-3-fe@dev.tdt.de


Signed-off-by: Lee Jones <lee@kernel.org>