net: dsa: mv88e6xxx: change serdes lane parameter type from u8 type to int

Returning 0 is no more an error case with MV88E6393 family
which has serdes lane numbers 0, 9 or 10.
So with this change .serdes_get_lane will return lane number
or -errno (-ENODEV or -EOPNOTSUPP).

Signed-off-by: Pavana Sharma <pavana.sharma@digi.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>