media: em28xx: fix a regression with HVR-950

Commit be7fd3c3 ("media: em28xx: Hauppauge DualHD second tuner
functionality") removed the logic with sets the alternate for the DVB
device. Without setting the right alternate, the device won't be
able to submit URBs, and userspace fails with -EMSGSIZE:

	ERROR     DMX_SET_PES_FILTER failed (PID = 0x2000): 90 Message too long

Tested with Hauppauge HVR-950 model A1C0.

Fixes: be7fd3c3

 ("media: em28xx: Hauppauge DualHD second tuner functionality")

Cc: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>