script: modpost: emit a warning when the description is missing

Emit a warning when the mod description is missed and only
when the W=1 is enabled.

Reported-by: Roland Kletzing <devzero@web.de>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=10770


Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Tested-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>