USB: usblp: fix use-after-free on disconnect

A recent commit addressing a runtime PM use-count regression, introduced
a use-after-free by not making sure we held a reference to the struct
usb_interface for the lifetime of the driver data.

Fixes: 9a315358 ("USB: usblp: fix runtime PM after driver unbind")
Cc: stable <stable@vger.kernel.org>
Reported-by:  <syzbot+cd24df4d075c319ebfc5@syzkaller.appspotmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191015175522.18490-1-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>