[media] vpfe_capture: fix compiler warning

davinci/vpfe_capture.c: In function 'vpfe_probe':
davinci/vpfe_capture.c:1992:9: warning: 'ret' may be used uninitialized
in this function [-Wmaybe-uninitialized]
   return ret;
          ^~~

This is indeed correct, so if the kmalloc fails set ret to -ENOMEM.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>