arcmsr: revise allocation of second dma_coherent_handle for type B

This modification is for consistency with upcoming adapter type D.
Both adapter type B and D have similar H/W and S/W structure.

Signed-off-by: Ching Huang <ching2048@areca.com.tw>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>