staging: fix comedi build errors

Fix build errors when CONFIG_COMEDI_PCI_DRIVERS is not enabled
and CONFIG_COMEDI_PCI_DRIVERS was used where CONFIG_ISA_DMA_API
should have been used.

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>