staging: comedi: cb_das16_cs: remove n_boards define

The 'n_boards' define is only used one place in the driver.
Just put the ARRAY_SIZE() where used and remove the define.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>