arcmsr: fix command timeout under heavy load

This patch rewrites the interrupt service routine relate function to fix
a command timeout under heavy controller load.

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