V4L/DVB (7312): pvrusb2: Indicate streaming status via LED

Most of this originates from Michael Krufky <mkrufky@linuxtv.org>;
these changes move LED control into separate functions.  This is the
first step in new work to make LED control a device-specific attribute.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>