staging: comedi: dt9812: tidy up the usb endpoint enumeration

Only the command_{write,read} usb endpoints are used by this driver.

Remove the message_pipe, and {read,write}_stream endpoint information
from the usb private data.

Rename the variables used for command endpoints to help keep the
lines < 80 chars.

Also, rename a couple of the local variables used to enumerate the
endpoints to also help keep the lines < 80 chars.

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>