staging: comedi: amplc_dio200_pci: no need to test board pointer in dio200_pci_detach()

`dio200_pci_detach()` doesn't need to check if the pointer to constant
board data (`thisboard`) and the pointer to private per-device data
(`devpriv`) are valid before calling `amplc_dio200_common_detach()`.  It
has no further need to check `thisboard` so remove the variable
altogether.  Move the test of `devpriv` to the first point it is needs
to be valid.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>