staging/comedi: fix the spaces issue around that '=' in drivers/ni_670x.c

fixed below checkpatch error.
- ERROR: spaces required around that '=' (ctx:WxV)

Signed-off-by: YAMANE Toshiaki <yamanetoshi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>