i40e: comment that udp_port must be in host byte order

The firmware expects the port number passed when setting up
the UDP tunnel configuration to be in Little Endian format.
The i40e_aq_add_udp_tunnel command byte swaps the value from
host order to Little Endian.

Since commit fe0b0cd9 ("i40e: send correct port number to
AdminQ when enabling UDP tunnels") we've correctly
sent the value in host order.

Let's also add a comment to the function explaining that it must
be in host order, as the port numbers are commonly stored as Big
Endian values.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>