staging: comedi: add 'ioenabled' flag to device

Add 1-bit bit-field member `ioenabled` of type `bool` to `struct
comedi_device`.  Use this to keep track of whether a PCI device and its
BARs have been successfully enabled by `comedi_pci_enable()`.  This
avoids overloading the meaning of the `iobase` member which is used by
several drivers to hold the base port I/O address of a board's "main"
registers.  Other drivers using MMIO use `iobase` as a flag to indicate
that the preceding call to `comedi_pci_enable()` was successful.  They
no longer need to do that.

The name `ioenabled` is intended to be PCI-agnostic so it can be used
for similar purposes by non-PCI drivers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>