media: dvb-frontends/cxd2099: Make en_templ constant

Static structure en_templ, of type dvb_ca_en50221, is not used except to
be copied into a local variable. Hence make it const to prevent
unintended modification of the original 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>