usb: diag_bridge: Modify kref_put handling in case of transfers

Make the code more readable by doing kref_put in the completion
callback if the URB is submitted successfully. Else do so from
the error handling in read/write functions.

Change-Id: I50645cac757293dd0b1df9afb356281b2922401b
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>