staging: comedi: comedi_pcmcia: introduce comedi_pcmcia_{enable, disable}

Introduce some helper functions to enable/disable the PCMCIA device.
This will allow removing some of the boilerplate code in the comedi
PCMCIA drivers.

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