V4L/DVB (13108): tda8290: enable deemphasis_50 module parameter

This adds a forgotten module_param macro needed to set a deemphasis of 50us.
It is the standard setting for commercial FM radio broadcasts outside the US.

Signed-off-by: Henk Vergonet <Henk.Vergonet@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>