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: 98a01119 ("net: sparx5: add compatible string for lan969x")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20241113115513.4132548-1-arnd@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>