Fixes bug where the app callback was getting the length of the status
request transfer rather than the length of the data stage payload.
TODO: Right now this returns the expected length, when it really should
be returning the transfer length.
This is really just a few descriptors and then answering to the
request from the host to reboot into DFU mode.
That latter part is delegated to the app since this is platform
specific.
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>