amd-xgbe: Fix maximum GPIO value check

The GPIO support in the hardware allows for up to 16 GPIO pins, enumerated
from 0 to 15.  The driver uses the wrong value (16) to validate the GPIO
pin range in the routines to set and clear the GPIO output pins.  Update
the code to use the correct value (15).

Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>