USB: RNDIS gadget, fix issues talking from PXA

The reworked Ethernet gadget has an RNDIS interop problem when used
with the CDC subset driver ... e.g. on PXA 2xx and 3xx hardware,
which currently has a hard time talking to MS-Windows hosts.

The issue is that Microsoft requires USB_CLASS_COMM.  Fix by tweaking
the CDC subset driver to not switch to USB_CLASS_VENDOR_SPEC if RNDIS
is used in some other device configuration.

[ UPDATED:  some "statements" were comma-terminated; fix that. ]

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Aric Blumer <aric@sdgsystems.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>