media: pvrusb2: fix parsing error

pvr2_std_str_to_id() returns 0 on failure and 1 on success,
however the caller is checking failure case using <0

Acked-by: Mike Isely <isely@pobox.com>
Co-developed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>