staging: comedi: adv_pci1760: Do not return EINVAL for CMDF_ROUND_DOWN.

The CMDF_ROUND_DOWN case falls through and so always returns -EINVAL.

Fixes: 14b93bb6 ("staging: comedi: adv_pci_dio: separate out PCI-1760 support")
Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>