usb-mtp: fix alignment of access of ObjectInfo filename field

usb-mtp: fix alignment of access of ObjectInfo filename field

The ObjectInfo struct's "filename" field is following a uint8_t
field in a packed struct and thus has bad alignment for a 16-bit
field. Switch the field to to uint8_t and use the helper function
for accessing unaligned 16-bit data.

Note that although the MTP spec specifies big endian, when transported
over the USB protocol, data is little endian.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20190415154503.6758-4-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 1259f27ec2113d34e54ee731aac5990d9606d9fa)