[media] s5p-mfc: add return value check in mfc_sys_init_cmd

alloc_dev_context_buffer method might fail, so add proper
return value check.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Kamil Debski <kamil@wypas.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>