media: uvcvideo: Add missing value for power_line_frequency

UVC 1.5 class defines 4 values for this control on:
4.2.2.3.6 Power Line Frequency Control

Add the missing value when the UVC version is 1.5.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>