[SCSI] vmw_pvscsi: Fix pvscsi_abort() function.

This change ensures that pvscsi_abort() function returns SUCCESS
only when the command in question was actually completed, otherwise
returns FAILURE. The code before change, was causing a bug where
driver tries to complete a command to the mid-layer while the mid-layer
has already requested the driver to abort that command, in response
to which the driver has responded with SUCCESS causing mid-layer
to free the command struct.

Signed-off-by: Arvind Kumar <arvindkumar@vmware.com>
Tested-by: Ewan Milne <emilne@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>