[media] solo6x10: fix potential null dereference

drivers/media/pci/solo6x10/solo6x10-disp.c:221 solo_set_motion_block() error: potential null dereference
'buf'.  (kzalloc returns null)

Also propagate this error up the chain.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>