gadgetfs: Fixed bug in ep_aio_read_retry.

I don't think the current code works with multiple iovecs.
The original would just copy the first part of priv->buf
over and over into multiple iovecs.

Signed-off-by: Sarah Bailey <saharabeara@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>