USB: fix serial gadget ACM breakage

Two of the CDC ACM control requests in the serial gadget have never
been correct, and have been reported to cause serious troubles ... as
in, soft lockup and maybe watchdog reset (depending on hardware).

This patch makes those request fail cleanly, rather than misbehaving.
Someone using CDC ACM should fix them according to the FIXME comments
which now replace the previous bugs.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>