staging: comedi: make ni_tio_has_gate2_registers return boolean

This patch makes ni_tio_has_gate2_registers return boolean, since
this function only uses either one or zero as its return value.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>