net: fill in MODULE_DESCRIPTION()s for dsa_loop_bdinfo

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the DSA loopback fixed PHY module.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20240208164244.3818498-10-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/dsa_loop_bdinfo.c

@@ -32,4 +32,5 @@ static int __init dsa_loop_bdinfo_init(void)
 }
 arch_initcall(dsa_loop_bdinfo_init)
 
+MODULE_DESCRIPTION("DSA mock-up switch driver");
 MODULE_LICENSE("GPL");