media: cxd2820r: remove unused cxd2820r_debug declaration

cxd2820r_debug has been removed since
commit 75aeafc9 ("[media] cxd2820r: switch to Kernel
dev_* logging"), so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>