staging: comedi: usbdux: consolidate the firmware upload

Absorb the usbdux_stop(), usbdux_upload(), and usbdux_start()
functions into firmware_upload().

Each of them just do a usb_control_msg() to the device and output
an error message if it fails. A similar message is also output by
firmware_upload() so the extra messages are redundant.

We can also share the malloc'ed local buffer needed for the
usb_control_msg().

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>