media: em28xx: Fix dual transport stream operation

Addresses the following, which introduced a regression itself:

Commit 509f8965 ("media: em28xx: fix a regression with HVR-950")

The regression fix breaks dual transport stream support. Currently,
when a tuner starts streaming it sets alt mode on the USB interface.
The problem is, in a dual tuner model, both tuners share the same
USB interface, so when the second tuner becomes active and sets alt
mode on the interface it kills streaming on the other port.

This patch addresses the regression by only setting alt mode
on the USB interface during em28xx_start_streaming, if the
device is not a dual tuner model. This allows all older and
single tuner devices to explicitly set alt mode during stream
startup. Testers report both isoc and bulk DualHD models work
correctly with the alt mode set only once, in em28xx_dvb_init.

Fixes: 509f8965 ("media: em28xx: fix a regression with HVR-950")
Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Michael Ira Krufky <mkrufky@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>