staging: comedi: ni_labpc: fix legacy driver build

CONFIG_COMEDI_NI_LABPC_ISA is a tristate option. #if IS_ENABLED()
needs to be used instead of #ifdef in order to build the legacy
specific code in this driver.

The local variable 'board' is not used in labpc_attach(). Remove it.

The labpc_detach() function is only referenced in this file. Make
it static.

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