net: fill in MODULE_DESCRIPTION()s for mpoa

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the Multi-Protocol Over ATM (MPOA) driver.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240208164244.3818498-3-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>