USB: dbgp gadget: set MaxpacketSize0

The current code doesn't set it, so linux complains about
it when connected, and ignores the device:

[104611.068082] usb 1-5: new high speed USB device using ehci_hcd and address 127
[104611.088368] usb 1-5: Invalid ep0 maxpacket: 0

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