ENA Driver: Remove invalid ring size alignment logic

[talawahtech]

This a backport of a fix for the ENA driver that was added in v21.05. Without this fix Seastar/DPDK apps can't run on AWS EC2 instances unless the default ring size is changed.

The previous logic was invalid. The RTE_ETH_DEV_FALLBACK_RX_RINGSIZE (and the TX counterpart) are values that rte_eth_rx_queue_setup() will set if dev_info.default_rxportconf.ring_size is 0 and user provided 0 in nb_rx_desc argument. However the current code treats it as a hint for the PMD to change the ring size to internal defaults.