ANDROID: GKI: sound/usb/card.h: add Android ABI padding to struct snd_usb_endpoint

There seems to be lots of people wanting to change this structure, and
while it feels internal, due to some SoC patches, there's an ABI that
needs to be preserved here over time.

So add some padding to struct snd_usb_endpoint so that we can maintain
this succesfully in the future.

Of course, now that the padding is there, there never will be any need
to change this structure again...

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I21cb3abbdca2961e33a94b2bc48b49f3fadbd206