Check return value from call to devm_kzalloc() in order to prevent
a NULL pointer dereference.

This issue was detected with the help of Coccinelle.

Fixes: 4a5f8ae5 ("[media] davinci: vpif_capture: get subdevs from DT when available")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>