uas: Urbs must be anchored before submitting them

Otherwise they may complete before they get anchored and thus never get
unanchored (as the unanchoring is done by the usb core on completion).

This commit also remove the usb_get_urb / usb_put_urb around cmd submission +
anchoring, since if done in the proper order this is not necessary.

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