scsi: storvsc: Fix a bug in the handling of SRB status flags

SRB status can have additional information. Mask these out before processing
SRB status.

This patch was sent as part of a collection of patches more than a year ago.
While the rest of the patches in the set were comitted, this patch was not.
I woulod like to thank Olaf for noticing that this patch was not committed
upstream.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>