amd-xgbe: Use proper Rx flow control register

Updated hardware documention shows the Rx flow control settings were
moved from the Rx queue operation mode register to a new Rx queue flow
control register.  The old flow control settings are now reserved areas
of the Rx queue operation mode register. Update the code to use the new
register.

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