staging: comedi: me_daq: fix pointer dereference in me_detach()

As pointed out by Dan Carpenter, dev_private could be NULL when it
is checked before calling comedi_pci_disable(). The private data
variable 'plx_regbase_size' is only used to see if the pci device
has been enabled.

Remove that variable from the private data and just use the unused
dev->iobase variable to let the detach know that the pci device
is enabled.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>