[media] s5p-tv: fix error return code in mxr_acquire_video()

Fix to return a negative error code in the vb2_dma_contig_init_ctx()
error handling case instead of 0, as done elsewhere in this function.
Also vb2_dma_contig_init_ctx() return ERR_PTR() in case of error and
never return NULL, so use IS_ERR() replace IS_ERR_OR_NULL().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>