dsa: mv88e6xxx: Fix unsigned/signed issue

dsa: mv88e6xxx: Fix unsigned/signed issue

commit dea870242a9c ("dsa: mv88e6xxx: Allow speed/duplex of port to be
configured") leads to the following static checker warning:

        drivers/net/dsa/mv88e6xxx.c:585 mv88e6xxx_adjust_link()
        warn: unsigned 'ret' is never less than zero.

drivers/net/dsa/mv88e6xxx.c
   573  void mv88e6xxx_adjust_link(struct dsa_switch *ds, int port,
   574                             struct phy_device *phydev)
   575  {
   576          struct mv88e6xxx_priv_state *ps = ds_to_priv(ds);
   577          u32 ret, reg;
   578
   579          if (!phy_is_pseudo_fixed_link(phydev))
   580                  return;
   581
   582          mutex_lock(&ps->smi_mutex);
   583
   584          ret = _mv88e6xxx_reg_read(ds, REG_PORT(port), PORT_PCS_CTRL);
   585          if (ret < 0)

Make ret an int, which is the return type for _mv88e6xxx_reg_read()

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>