net: fill in MODULE_DESCRIPTION()s for CAIF

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to all the CAIF sub-modules.

Link: https://lore.kernel.org/r/20240104144855.1320993-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>