staging: comedi: ni_65xx: add ni_65xx_num_ports()

Add a function to return the total number of digital I/O, digital input,
and digital output ports on the board.  Each port has 8 channels (bits).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>