staging: comedi: ni_usb6501: add counter subdevice

Add counter support for NI USB-6501.

The following functions are introduced:
- ni6501_counter_command()
- ni6501_cnt_insn_config()
- ni6501_cnt_insn_read()
- ni6501_cnt_insn_write()

Signed-off-by: Luca Ellero <luca.ellero@brickedbrain.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>