staging: comedi: dt9812: tidy up usb_bulk_msg() calls

To clarify the code, add a local variable for the struct usb_device
pointer used in the usb_blk_msg() calls.

It's not necessary to initialize the 'count' when writing to the
usb device. The 'count' variable is used to get back the number
of bytes actually sent.

Just return the usb_blk_msg() result when it is the last operation
in a function.

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>