LinuxKPI: extend struct and enum for leds

An updated rtw88 driver requires minimal changes to leds.h. On
the positive side we can now remove the __DUMMY from the enum as we
actually have a valid field name.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50002