Implement use of secondary TS port on em28xx.
Adds has_dual_ts field, allows secondary demod/tuner to be
added to a single em28xx device.

Hauppauge DualHD models are configured to use this feature.

[mchehab@s-opensource.com: em28xx_duplicate_dev() should be static]
Signed-off-by: Brad Love <brad@nextdimension.cc>
Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>