staging: comedi: dt9812: move the sempaphore used by the comedi_driver

The semaphore in the struct slot_dt9812 is used by the comedi_driver
when reading and writing to the usb device. It has no real association
with the 'slot'. The 'slot' is protected with a static semaphore in
the driver.

Move the semaphore into the comedi_device private data and initialize
it after the private data is allocated.

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>