[media] cx88: don't use current_norm

current_norm can only be used if there is a single device node since it is
local to the device node. In this case multiple device nodes share a single
tuner.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>