USB: Fix error cleanup path in airprime

Fix up the error processing path: in usb_submit_urb failed, we forgot
to free buffers. Also, don't free buffers in read callback: less error
prone, 21 LOC less, no need to comment so much. N.B. write path is ok
to do kfree.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>