[media] mantis: Fix error handling in mantis_dma_init()

Current code assigns 0 to variable 'err', which makes mantis_dma_init()
to return success even if mantis_alloc_buffers() fails.

Fix it by checking the return value from mantis_alloc_buffers() and
propagating it in the case of error.

Reported-by: RUC_Soft_Sec <zy900702@163.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>