net/enic: initialize RQ fetch index before enabling RQ

net/enic: initialize RQ fetch index before enabling RQ

The fetch index must be initialized only when RQ is
disabled. Otherwise, it may lead to stale entries in IG descriptor
cache on the VIC.

Fixes: a74629cfa3a1 ("net/enic: enable RQ first and then post Rx buffers")
Cc: stable@dpdk.org
Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>