media: solo6x10: Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
./drivers/media/pci/solo6x10/solo6x10-core.c:371:60-61: WARNING
opportunity for kobj_to_dev()

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>