staging: comedi: fix build errors caused by module_init/module_exit refactor

A couple build errors were introduced with the module_init/module_exit
refactor.

The struct comedi_driver variable was being accessed directly in the
attach and detach routines. Instead of doing this, access the variable
indirectly using the driver pointer in struct comedi_device.

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