staging: fix comedi build when ISA_DMA_API is enabled but COMEDI_PCI is not enabled

Fix build when CONFIG_ISA_DMA_API is enabled but
CONFIG_COMEDI_PCI[_DRIVERS] is not enabled.

Fixes these build errors:

  drivers/staging/comedi/drivers/ni_labpc.c: In function 'labpc_ai_cmd':
  drivers/staging/comedi/drivers/ni_labpc.c:1351: error: implicit declaration of function 'labpc_suggest_transfer_size'
  drivers/staging/comedi/drivers/ni_labpc.c: At top level:
  drivers/staging/comedi/drivers/ni_labpc.c:1802: error: conflicting types for 'labpc_suggest_transfer_size'
  drivers/staging/comedi/drivers/ni_labpc.c:1351: note: previous implicit declaration of 'labpc_suggest_transfer_size' was here

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>