scsi: core: scsi_get_device_flags_keyed(): Always return device flags

Since scsi_get_device_flags_keyed() callers do not check whether or not
the returned value is an error code, change that function such that it
returns a flags value even if the 'key' argument is invalid.  Note:
since commit 28a0bc41 ("scsi: sd: Implement blacklist option for
WRITE SAME w/ UNMAP") bit 31 is a valid device information flag so
checking whether bit 31 is set in the return value is not sufficient to
tell the difference between an error code and a flags value.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>