arcmsr: clear outbound doorbell buffer completely

Clear outbound doorbell buffer completely for adapter type C.  This is to
prevent getting bad data input from IOP before ioctl command processing
starts.

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