arcmsr: Split dma resource allocation to a new function

Split dma resource allocation and io register assignment from get_config
to a new function arcmsr_alloc_io_queue.

Signed-off-by: Ching Huang <ching2048@areca.com.tw>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>