net/mlx5e: Update xoff formula

[ Upstream commit 5ec983e924c7978aaec3cf8679ece9436508bb20 ]

Set minimum speed in xoff threshold formula to 40Gbps

Fixes: 0696d608 ("net/mlx5e: Receive buffer configuration")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>