usb: gadget: f_diag: Expose DLOAD pid/serial entries to configfs

Allow for userspace to directly write the PID/serial entries to
the DIAG DLOAD cookie IMEM location by exposing configfs
attributes under the f_diag driver. While at it, flatten the
definition of the dload structure for clarity.

Change-Id: Ic5049835beca532a430b0446a2126e5f7818b4bb
Signed-off-by: Jack Pham <jackp@codeaurora.org>