amd-xgbe: Reported fifo size from hardware is not correct

The fifo size reported by the hardware is not correct. Add support
to limit the reported size to what is actually present.  Also, fix
the argument types used in the fifo size calculation function.

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