amd-xgbe: Simplify calculation and setting of queue fifos

The calculation of the Tx and Rx fifo sizes can be calculated rather
than hardcoded in a switch statement. Additionally, the per-queue fifo
sizes can be calculated rather than hardcoded using if/else if statements
that can possibly underutilize the available fifo area.

Change the code to calculate the fifo sizes and the per-queue fifo sizes
to simplify the code and make best use of the available fifo.

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