media: am437x-vpfe: Make use of to_vpfe() to get a pointer to vpfe_device

This patch makes use of to_vpfe() to get a pointer to vpfe_device

Signed-off-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>