CHROMIUM: gobi: Port dynamic endpoint discovery (2 of 2)

CHROMIUM: gobi: Port dynamic endpoint discovery (2 of 2)

Actually use the discovered interface number and interrupt endpoint.

Also, rewrite int_callback so it doesn't treat the entire USB packet header
as a single u64, and so it pays even a little attention to endianness.

BUG=chromium-os:18547
TEST=suite_Cellular passes (except ConnmanCromoCrash, which always fails)

Signed-off-by: ttuttle <ttuttle@chromium.org>
Change-Id: I51ed85ad55e9a73161721199793efac353b83b93
Reviewed-on: http://gerrit.chromium.org/gerrit/7289
Reviewed-by: Elly Jones <ellyjones@chromium.org>
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Tested-by: ttuttle <ttuttle@chromium.org>