[media] gspca: ov534/topro: use a define for the default framerate

When writing the change in commit dcc7fdbe ("[media] gspca:
ov534/topro: prevent a division by 0") I used magic numbers for the
default framerate to minimize the code footprint to make it easier to
backport the patch to the stable trees.

However it's better if the default framerate has its own define to avoid
risking using different values in different places, and for readability.

While at it also remove some trivial comments about the framerates which
don't add much to the code anymore.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>