V4L/DVB (8462): cx18: Lock the aux PLL to the video pixle rate for analog captures

cx18: Lock the aux PLL to the video pixel rate for analog captures.  The
datasheet for the CX25840 says this is important for MPEG encoding applications.
To ensure the PLL locking was correct, also fixed the aux PLL's multiplier to
be computed based on a precise crystal freq of 4.5 MHz/286 * 455/2 * 8 =
28636363.6363... instead of the imporperly rounded 28636363.

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