rswitch: Use unsigned int for port related array index

Array index should not be negative, so modify the condition of
rswitch_for_each_enabled_port_continue_reverse() macro, and then
use unsigned int instead.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>