V4L/DVB (9514): cx18: Fix PLL freq computation for debug display

cx18: Fix PLL freq computation for debug display.
The code to compute the PLL freq from register values was storing an
intermediate 56 bit result in a 32 bit type, causing a nonsense value to
be displayed.

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