staging: comedi: ni_tio: remove BUG() in ni_tio_set_gate_src()

This BUG() can never happen. The previous ni_tio_has_gate2_registers()
check will have already caused the function to return -EINVAL.

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>