leds: pwm: remove header

The header is only used by leds_pwm.c, so move contents to leds_pwm.c
and remove it.
Apply minor changes suggested by checkpatch.
Remove deprecated and unused pwm_id member.

Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Denis Osterland-Heim <Denis.Osterland@diehl.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>