phy: marvell/micrel: Fix Unpossible condition

commit 2b2427d0 ("phy: micrel: Add ethtool statistics counters")
from Dec 30, 2015, leads to the following static checker
warning:

        drivers/net/phy/micrel.c:609 kszphy_get_stat()
        warn: unsigned 'val' is never less than zero.

drivers/net/phy/micrel.c
   602  static u64 kszphy_get_stat(struct phy_device *phydev, int i)
   603  {
   604          struct kszphy_hw_stat stat = kszphy_hw_stats[i];
   605          struct kszphy_priv *priv = phydev->priv;
   606          u64 val;
   607
   608          val = phy_read(phydev, stat.reg);
   609          if (val < 0) {
                    ^^^^^^^
Unpossible!

   610                  val = UINT64_MAX;
   611          } else {
   612                  val = val & ((1 << stat.bits) - 1);
   613                  priv->stats[i] += val;
   614                  val = priv->stats[i];
   615          }
   616
   617          return val;
   618  }

The same problem exists in the Marvell driver. Fix both.

Fixes: 2b2427d0 ("phy: micrel: Add ethtool statistics counters")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Julia.Lawall <julia.lawall@lip6.fr>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>