staging: comedi: ni_atmio: fix build errors

The following commits introduced a couple build errors in this driver
due to the removal of some macros in ni_stc.h.

commit: f5a1d92b "staging: comedi: ni_stc.h: remove n_ni_boards macro"
commit: 6293e357 "staging: comedi: ni_stc.h: remove boardtype macro"

The n_ni_boards macro is an open coded version of ARRAY_SIZE.

The boardtype macro is removed in favor of using the comedi_board()
helper and accessing the boardinfo with a pointer.

Fix both issues.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>