net/enic: fix uninitialized variable

net/enic: fix uninitialized variable

A local variable was used without initialization and triggered a
coverity issue.

Is is fixed here, but there is no ill effect of not initializing
the variable in this case. 'rxq_interrupt_offset' is irrelevant
if 'rxq_interrupt_enable' is not set (the condition caught by
coverity).

Coverity issue: 268314
Fixes: fc2c8c0668fd ("net/enic: use Tx completion index instead of messages")
Cc: stable@dpdk.org
Signed-off-by: John Daley <johndale@cisco.com>
Reviewed-by: Hyong Youb Kim <hyonkim@cisco.com>