[media] r820t: fix size and init values

Correct the initialization values at the start of the function
and use proper variable sizes to prevent overflow.

Signed-off-by: Benjamin Larsson <benjamin@southpole.se>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>