ixgbe: fix statistics for 82598/82599 differences

ixgbe: fix statistics for 82598/82599 differences

Ixgbe based 82598 and 82599 have different priority receive link-on
register addresses. This is solved in base/ by providing in the
PXONRXC and PXONXCNT as separate macros. This patch ensures the
correct address is read, avoiding reading garbage values.

Also PXON2OFFCNT doesn't exist in 82598, so it is not read for
that MAC.

This issue has existed since the drivers were imported into DPDK,
but was not easily discoverable as xstats were not available.
Tested using testpmd> show port xstats all

Fixes: af75078fece3 ("first public release")

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>