ethdev: add comment to warn of ABI breakage

ethdev: add comment to warn of ABI breakage

If a function is added to the eth_dev_ops struct before
tx_descriptor_status function, this will cause ABI breakage. This is due
to static inline functions using this function, and some other functions
above it in the struct, so they cannot change position. A comment is
added to inform developers of this possible breakage.

Signed-off-by: Ciara Power <ciara.power@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>