dsa: change "select" to "depends on" for NET_SWITCHDEV and for NET_DSA

This would fix randconfig compile error:
net/built-in.o: In function `netdev_switch_fib_ipv4_abort':
(.text+0xf7811): undefined reference to `fib_flush_external'

Also it fixes following warnings:
warning: (NET_DSA) selects NET_SWITCHDEV which has unmet direct dependencies (NET && INET)

warning: (NET_DSA_MV88E6060 && NET_DSA_MV88E6131 && NET_DSA_MV88E6123_61_65 && NET_DSA_MV88E6171 && NET_DSA_MV88E6352 && NET_DSA_BCM_SF2) selects NET_DSA which has unmet direct dependencies (NET && HAVE_NET_DSA && NET_SWITCHDEV)

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>