staging: comedi: ni_660x: remove ni_660x_num_counters()

This inline function is only used by the (*auto_attach). Remove it
and just use a local variable for the calculation.

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