mlxsw: switchib: add MLXSW_PCI dependency

The newly added switchib driver fails to link if MLXSW_PCI=m:

drivers/net/ethernet/mellanox/mlxsw/mlxsw_switchib.o: In function^Cmlxsw_sib_module_exit':
switchib.c:(.exit.text+0x8): undefined reference to `mlxsw_pci_driver_unregister'
switchib.c:(.exit.text+0x10): undefined reference to `mlxsw_pci_driver_unregister'
drivers/net/ethernet/mellanox/mlxsw/mlxsw_switchib.o: In function `mlxsw_sib_module_init':
switchib.c:(.init.text+0x28): undefined reference to `mlxsw_pci_driver_register'
switchib.c:(.init.text+0x38): undefined reference to `mlxsw_pci_driver_register'
switchib.c:(.init.text+0x48): undefined reference to `mlxsw_pci_driver_unregister'

The other two such sub-drivers have a dependency, so add the same one
here. In theory we could allow this driver if MLXSW_PCI is disabled,
but it's probably not worth it.

Fixes: d1ba5263 ("mlxsw: switchib: Introduce SwitchIB and SwitchIB silicon driver")
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>