media: dib7000p: Fix potential division by zero

Variable loopdiv can be assigned 0, then it is used as a denominator,
without checking it for 0.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 713d54a8 ("[media] DiB7090: add support for the dib7090 based")
Signed-off-by: Daniil Dulov <d.dulov@aladdin.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: (bw != NULL) -> bw]