ethdev: add sanity checks to functions

ethdev: add sanity checks to functions

The functions rte_eth_rx_queue_count and rte_eth_descriptor_done are
supported by very few PMDs. Therefore, it is best to check for support
for the functions in the ethdev library, so as to avoid run-time crashes
at run-time if the application goes to use those APIs. Similarly, the
port parameter should also be checked for validity.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>