media: vivid: remove redundant assignment to variable checksum

Variable checksum is assigned a value that is never read, it is assigned
a new value in a following for-loop. The assignment is redundant and can
be removed.

Cleans up clang scan build warning:
drivers/media/test-drivers/vivid/vivid-vbi-gen.c:197:2: warning: Value
stored to 'checksum' is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>