USB: dbgp gadget: fix return value of dbgp_setup

Current code returns 0 even if it can't handle the request.
This leads to timeouts when an unhandled request is sent:

Bus 001 Device 003: ID 0525:c0de Netchip Technology, Inc.
Device Descriptor:
[..]
can't get device qualifier: Connection timed out
[..]

change the code to return EOPNOTSUPP in such cases.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>