staging: comedi: addi_apci_3120: remove check in apci3120_setup_chan_list()

The n_chan check is not needed. This value will always be >= 1. Remove
the unnecessary check.

For aesthetics, rename the function and change it's return type to void.

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>