staging: comedi: addi_apci_1564: remove null check of devpriv in apci1564_detach()

There is no need to test whether devpriv is null in this function.  The
check looks left over and we can just remove it.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>