V4L/DVB (8461): cx18: Fix 32 kHz audio sample output rate for analog tuner SIF input

cx18: Fix 32 kHz audio sample output rate for analog tuner SIF input so it
works.  The AUX_PLL VCO was being operated at 196.6 MHz out of the spec'ed
200-600 MHz range.  Fixed the multipler and post dividers to operate the VCO
within specification and added comments on how magic numbers are derived.

Thanks to Hans Verkuil for pointing out this interesting problem to solve.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>