media: vpbe_display: properly handle error case

if v4l2_subdev_call(..., VENC_GET_FLD,...) fails, it
currently returns a random value. Instead, return 1.

That's probably better than returning 0, as this is very
likely what happens in practice with the current code, as
as the probably of an unititialized 32 bits integer to
have an specific value (0, in this case), is 1/(2^32).

An alternative would be to return an error code, and
let the caller to hint, based on the past received
frame, but that sounds weird.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>