staging: comedi: refactor sysfs files in comedi_fops.c

Refactor the sysfs attributes and functions to remove
the need for the forward declarations and use the
DEVICE_ATTR macro to define them.

Instead of individually creating sysfs device attribute
files, wrap them in an attribute_group and use the
sysfs_create_group function to create them.

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>