amd-xgbe: Treat certain counter registers as 64 bit

Even if the management counters are configured to be 32 bit register
values, the [rt]xoctetcount_gb and [rt]xoctetcount_g counters are
always 64 bit counter registers.  Since they are not being treated as
64 bit values, these statistics are being reported incorrectly (ifconfig,
ethtool, etc.).

Update the routines used to read the registers to access the "hi"
register (an offset of 4 from the "lo" register) to create a 64 bit
value for these 64 bit counters.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>