diag: Do not USB write_done if MHI has gone down

Race condition in handling MHI read channel buffers while external
proc has gone down and USB connect and disconnect events is prevented
by not freeing the read channel buffer when USB has notified write_done
if MHI has gone down.

Change-Id: I725b5115947390c7659047ac6848a797e1854a93
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>