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>