usb: gadget: Add snapshot of USB DIAG function driver

This change adds USB DIAG function driver to support diagnostics
functionality with USB transport. This snapshot is taken as of
msm-4.4 commit 4fa8f96ae60c ("soc: qcom: spss_utils: fix
test_fuse_state_show()").

- There is no android.c file which was sharing dload related data
structure. Hence this change removes diag_dload.h file and adds
required dload specific support with USB diag driver.

Change-Id: Ia7ce036bac36161d470c98f7572f9cbcb102b721
Signed-off-by: Mayank Rana <mrana@codeaurora.org>