net: dsa: mv88e6xxx: fix boolreturn.cocci warnings

drivers/net/dsa/mv88e6xxx/serdes.c:66:9-10: WARNING: return of 0/1 in function 'mv88e6352_port_has_serdes' with return type bool

 Return statements in functions returning bool should use
 true/false instead of 1/0.
Generated by: scripts/coccinelle/misc/boolreturn.cocci

Fixes: eb755c3f ("net: dsa: mv88e6xxx: Add helper to determining if port has SERDES")
CC: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>