net: dsa: ocelot: add NET_VENDOR_MICROSEMI dependency

net: dsa: ocelot: add NET_VENDOR_MICROSEMI dependency

Selecting MSCC_OCELOT_SWITCH is not possible when NET_VENDOR_MICROSEMI
is disabled:

WARNING: unmet direct dependencies detected for MSCC_OCELOT_SWITCH
  Depends on [n]: NETDEVICES [=y] && ETHERNET [=n] && NET_VENDOR_MICROSEMI [=n] && NET_SWITCHDEV [=y] && HAS_IOMEM [=y]
  Selected by [m]:
  - NET_DSA_MSCC_FELIX [=m] && NETDEVICES [=y] && HAVE_NET_DSA [=y] && NET_DSA [=y] && PCI [=y]

Add a Kconfig dependency on NET_VENDOR_MICROSEMI, which also implies
CONFIG_NETDEVICES.

Depending on a vendor config violates menuconfig locality for the DSA
driver, but is the smallest compromise since all other solutions are
much more complicated (see [0]).

https://www.spinics.net/lists/netdev/msg618808.html

Fixes: 56051948773e ("net: dsa: ocelot: add driver for Felix switch family")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>