net: Always descend into dsa/ folder with CONFIG_NET_DSA enabled

Stephen reported that he was unable to get the dsa_loop driver to get
probed, and the reason ended up being because he had CONFIG_FIXED_PHY=y
in his kernel configuration. As Masahiro explained it:

  "obj-m += dsa/" means everything under dsa/ must be modular.

  If there is a built-in object under dsa/ with CONFIG_NET_DSA=m,
  you cannot do  "obj-$(CONFIG_NET_DSA) += dsa/".

  You need to change it back to "obj-y += dsa/".

This was the case here whereby CONFIG_NET_DSA=m, and so the
obj-$(CONFIG_FIXED_PHY) += dsa_loop_bdinfo.o rule is not executed and
the DSA loop mdio_board info structure is not registered with the
kernel, and eventually the device is simply not found.

To preserve the intention of the original commit of limiting the amount
of folder descending, conditionally descend into drivers/net/dsa when
CONFIG_NET_DSA is enabled.

Fixes: 227d7206 ("dsa: simplify Kconfig symbols and dependencies")
Reported-by: Stephen Langstaff <stephenlangstaff1@gmail.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>