media: cetrus: return an error if alloc fails

As warned by smatch:

	drivers/staging/media/sunxi/cedrus/cedrus.c: drivers/staging/media/sunxi/cedrus/cedrus.c:93 cedrus_init_ctrls() error: potential null dereference 'ctx->ctrls'.  (kzalloc returns null)

While here, remove the memset(), as kzalloc() already zeroes the
struct.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>