net: dsa: mv88e6xxx: Add missing initialization in mv88e6xxx_set_port_state()

drivers/net/dsa/mv88e6xxx.c: In function ‘mv88e6xxx_set_port_state’:
drivers/net/dsa/mv88e6xxx.c:905: warning: ‘ret’ may be used uninitialized in this function

If oldstate == state, mv88e6xxx_set_port_state() will return an
uninitialized value. Pre-initialize ret to zero to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>