net: dsa: mv88e6xxx: use gpio get optional variant

Use the optional variant to get the reset GPIO line, instead of checking
for the -ENOENT error.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>