media: ec168: Make structure ec168_props constant

The static structure ec168_props, of type dvb_usb_device_properties, is
used only once, when it is assigned to field props of the constant
structure ec168_driver_info. As this means that ec168_props will never be
modified, make it const as well in order to prevent unintended
modification of its fields.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>