staging: comedi: labpc: fix isadma dependency

When COMEDI_NI_LABPC is built-in and COMEDI_NI_LABPC_ISA is a loadable
module, thhe ISA DMA code is not reachable by the common module, causing
a link error:

drivers/staging/built-in.o: In function `labpc_interrupt':
ni_labpc_common.c:(.text+0x1d178): undefined reference to `labpc_handle_dma_status'
ni_labpc_common.c:(.text+0x1d1cb): undefined reference to `labpc_drain_dma'
drivers/staging/built-in.o: In function `labpc_ai_cmd':
ni_labpc_common.c:(.text+0x1d8ad): undefined reference to `labpc_setup_dma'

This changes the definition of COMEDI_NI_LABPC_ISADMA so that it will
also be builtin for that case. This looks like a rather old bug, but
I have never seen this in randconfig testing until today.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>