uas: Fix response iu struct definition

The response iu struct before this patch has a size of 7 bytes (discounting
padding), which is weird since all other iu-s are explictly padded to
a multiple of 4 bytes.

More over submitting a 7 byte bulk transfer to the status endpoint when
expecting a response iu results in an USB babble error, as the device
actually sends 8 bytes.

Up on closer reading of the UAS spec:
http://www.t10.org/cgi-bin/ac.pl?t=f&f=uas2r00.pdf

The reason for this becomes clear, the 2 entries in "Table 17 — RESPONSE IU"
are numbered 4 and 6, looking at other iu definitions in the spec, esp.
multi-byte fields, this indicates that the ADDITIONAL RESPONSE INFORMATION
field is not a 2 byte field as one might assume at a first look, but is
a multi-byte field containing 3 bytes.

This also aligns with the SCSI Architecture Model 4 spec, which UAS is based
on which states in paragraph "7.1 Task management function procedure calls"
that the "Additional Response Information" output argument for a Task
management function procedure call is 3 bytes.

Last but not least I've verified this by sending a logical unit reset task
management call with an invalid lun to an actual uasp device, and received
back a response-iu with byte 6 being 0, and byte 7 being 9, which is the
responce code for an invalid iu, which confirms that the response code is
being reported in byte 7 of the response iu rather then in byte 6.

Things were working before despite this error in the response iu struct
definition because the additional response info field is normally filled
with zeros, and 0 is the response code value for success.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

include/linux/usb/uas.h

@@ -83,7 +83,7 @@ struct response_iu {
 	__u8 iu_id;
 	__u8 rsvd1;
 	__be16 tag;
-	__be16 add_response_info;
+	__u8 add_response_info[3];
 	__u8 response_code;
 };