[media] cx25840: fix vsrc/hsrc usage on cx23888 designs

The location of the vsrc/hsrc registers moved in the cx23888, causing
the s_mbus call to fail prematurely indicating that "720x480 is not a
valid size". The function bailed out before many pertinent registers
were set related to the scaler (causing unexpected results in video
rendering when doing raw video capture).

Use the correct registers for the cx23888.

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>