eventdev/eth_rx: fix stalls on event device backpressure

eventdev/eth_rx: fix stalls on event device backpressure

In the Eventdev Ethernet RX Adapter, correctly handle the case where
the circular enqueue buffer head and last index point to the same
element.

This bug may be triggered in case there is backpressure from the event
device to the RX adapter.

Fixes: 8113fd15e229 ("eventdev/eth_rx: make enqueue buffer circular")
Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
Acked-by: Jay Jayatheerthan <jay.jayatheerthan@intel.com>