staging: comedi: comedi_fops: rename comedi_{read,write}_subdevice()

The 'get' usually implies increasing a reference count of an object.
These function return a pointer but do not do any reference counting.
For aesthetic reasons, rename the functions to better represent what
they do.

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>