net: dsa: fix build error with devlink build as module

After commit 96567d5d ("net: dsa: dsa2: Add basic support of devlink")
I see the following link error with CONFIG_NET_DSA=y and CONFIG_NET_DEVLINK=m:

net/built-in.o: In function 'dsa_register_switch':
(.text+0xe226b): undefined reference to `devlink_alloc'
net/built-in.o: In function 'dsa_register_switch':
(.text+0xe2284): undefined reference to `devlink_register'
net/built-in.o: In function 'dsa_register_switch':
(.text+0xe243e): undefined reference to `devlink_port_register'
net/built-in.o: In function 'dsa_register_switch':
(.text+0xe24e1): undefined reference to `devlink_port_register'
net/built-in.o: In function 'dsa_register_switch':
(.text+0xe24fa): undefined reference to `devlink_port_type_eth_set'
net/built-in.o: In function 'dsa_dst_unapply.part.8':
dsa2.c:(.text.unlikely+0x345): undefined reference to 'devlink_port_unregister'
dsa2.c:(.text.unlikely+0x36c): undefined reference to 'devlink_port_unregister'
dsa2.c:(.text.unlikely+0x38e): undefined reference to 'devlink_port_unregister'
dsa2.c:(.text.unlikely+0x3f2): undefined reference to 'devlink_unregister'
dsa2.c:(.text.unlikely+0x3fb): undefined reference to 'devlink_free'

Fix this by adding a dependency on MAY_USE_DEVLINK so that CONFIG_NET_DSA
get switched to be build as module when CONFIG_NET_DEVLINK=m.

Fixes: 96567d5d ("net: dsa: dsa2: Add basic support of devlink")
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>