staging: comedi: drivers (core): don't BUG_ON due to faulty drivers

The postconfig for drivers that support async commands currently can
BUG_ON if the subdevice was improperly configured by the driver.

Change the BUG_ON so that a dev_warn() is output and the postconfig
returns -EINVAL. This will prevent the comedi core from attaching to
the faulty driver but does not BUG the kernel.

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