amd-xgbe: Use the Tx queue count for Tx flow control support

When configuring Tx flow control the Rx queue count was used instead of
the Tx queue count for looping through the Tx hardware queues. Fix the
code to use the Tx queue count.

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