net: sky2: initialize return of gm_phy_read

clang static analysis flags this garbage return

drivers/net/ethernet/marvell/sky2.c:208:2: warning: Undefined or garbage value returned to caller [core.uninitialized.UndefReturn]
        return v;
        ^~~~~~~~

static inline u16 gm_phy_read( ...
{
	u16 v;
	__gm_phy_read(hw, port, reg, &v);
	return v;
}

__gm_phy_read can return without setting v.

So handle similar to skge.c's gm_phy_read, initialize v.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>