net/i40e/base: comment that UDP port must be in host order

The firmware expects the Port number to be in Little Endian format, and
the i40e_aq_add_udp_tunnel command clearly expects the udp_port variable
to be in Host order, as it uses CPU_TO_LE16(). It was recently
discovered in the Linux driver that we were passing a Big Endian port
number, which was therefor not enabling the UDP tunnel correctly.

Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>