net: sparx5: add missing lan969x Kconfig dependency

The sparx5 switchdev driver can be built either with or without support
for the Lan969x switch. However, it cannot be built-in when the lan969x
driver is a loadable module because of a link-time dependency:

arm-linux-gnueabi-ld: drivers/net/ethernet/microchip/sparx5/sparx5_main.o:(.rodata+0xd44): undefined reference to `lan969x_desc'

Add a Kconfig dependency to reflect this in Kconfig, allowing all
the valid configurations but forcing sparx5 to be a loadable module
as well if lan969x is.

Fixes: 98a01119608d ("net: sparx5: add compatible string for lan969x")
Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Daniel Machon <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/microchip/lan969x/Kconfig

···
 config LAN969X_SWITCH
-	tristate "Lan969x switch driver"
+	bool "Lan969x switch driver"
 	depends on SPARX5_SWITCH
 	help
 	  This driver supports the lan969x family of network switch devices.

drivers/net/ethernet/microchip/lan969x/Makefile

···
 # Makefile for the Microchip lan969x network device drivers.
 #
 
-obj-$(CONFIG_LAN969X_SWITCH) += lan969x-switch.o
+obj-$(CONFIG_SPARX5_SWITCH) += lan969x-switch.o
 
 lan969x-switch-y := lan969x_regs.o lan969x.o lan969x_calendar.o \
  lan969x_vcap_ag_api.o lan969x_vcap_impl.o