staging: comedi: ni_at_a2150: range check board index

The "ni_at_a2150" driver determines the board type by calling
`a2150_probe()`.  This reads a register and converts it to a board index
in the range 0 to 3.  However, the board table array it indexes into
(`a2150_boards[]`) only has 2 entries.  Return an error from the
Comedi driver "attach" handler `a2150_attach()` if the probed board
index is beyond the end of the array.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>