net/i40e: fix stats counters

net/i40e: fix stats counters

When low and high registers are read separately, this opens the door to
a race condition:
- low register is read
- NIC updates the registers
- high register is read

Because of this, we may end up with an incorrect counter value.
Let's read the registers in one shot, as it is done in Linux kernel
since the introduction of the i40e driver.

Fixes: 4861cde46116 ("i40e: new poll mode driver")
Cc: stable@dpdk.org
Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>