platform/x86: mlx-platform: Use define for the channel numbers

Add define for the channels number for mux device, instead of using
hardcoded value inside the code in order to improve code readability.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>