staging: comedi: split out PCI support into new module

staging: comedi: split out PCI support into new module

Setting the `CONFIG_COMEDI_PCI_DRIVERS` kernel configuration option
makes the main "comedi" module depend on the PCI support in the kernel.
That's not that big a deal since PCI support in the kernel is either
built into the kernel or is absent, and is not in a separate module.
Still, not all low-level Comedi drivers need PCI support, so we could
save a bit of space by not including it.  The Comedi PCI support
functions are all in "comedi_pci.c".  Turn it into a separate module so
the support code doesn't have to be loaded unnecessarily.

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