[media] cx25840: fix regression in HVR-1800 analog audio

The refactoring of the cx25840 driver to support the cx23888 caused breakage
with the existing support for cx23885/cx23887 analog audio support.  Tweak
the code so that it only uses the code if it really is a cx23888 instead of
applying it to all cx2388x based devices.

Validated with the following boards:

HVR-1800 retail (0070:7801)
HVR-1800 OEM (0070:7809)
HVR_1850 retail	(0070:8541)

Thanks to Steven Toth and Hauppauge for	loaning	me various boards to
regression test with.

Reported-by: Jonathan <sitten74490@mypacks.net>
Thanks-to: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Devin Heitmueler <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>