staging: comedi: comedi_bond: just check devprivs->devs once on detach

The `while` loop in `bonding_detach()` doesn't need to check
`devpriv->devs` each time round the loop.  Move the test outside the
loop.  The enclosing `if (devpriv)` can be changed to `if (devpriv &&
devpriv->devs)` as everything in this `if` statement is associated with
`devpriv->devs` anyway.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>