usb: usbtmc: Fix disconnect/poll interaction

When the device is disconnected poll waiters were not being woken.

Changes for v2:
  - add commit summary
  - add Fixes and Reported-by tags

Fixes: eb6b92ec ("Add support for receiving USBTMC USB488 SRQ notifications via poll/select")
Reported-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>