net: dsa: Fix duplicate object rule

While adding switch.o to the list of DSA object files, we essentially
duplicated the previous obj-y line and just added switch.o, remove the
duplicate.

Fixes: f515f192 ("net: dsa: add switch notifier")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/Makefile

 # the core
 obj-$(CONFIG_NET_DSA) += dsa_core.o
-dsa_core-y += dsa.o slave.o dsa2.o
 dsa_core-y += dsa.o slave.o dsa2.o switch.o
 
 # tagging formats