media: pwm-ir-tx: Trigger edges from hrtimer interrupt context

commit 363d0e56 upstream.

This makes the generated IR much more precise. Before this change, the
driver is unreliable and many users opted to use gpio-ir-tx instead.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>