net: aquantia: fix var initialization warning

found by sparse, simply useless local initialization with zero.

Fixes: 94ad9455 ("net: aquantia: add PTP rings infrastructure")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>