drivers: dma-coherent: simplify dma_init_coherent_memory return value

Since only dma_declare_coherent_memory cares about
dma_init_coherent_memory returning part of flags as it return value,
move the condition to the former and simplify the latter.  This in
turn makes rmem_dma_device_init less confusing.

Reported-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>