usb: misc: diag_bridge: use platform_device_register_simple

Simplify platform_device allocation in probe() function by
calling platform_device_register_simple(). This does the job
of both allocation as well as calling platform_device_add(),
which will reduce the number of error checks needed by one.

CRs-fixed: 534397
Change-Id: I02815b67a36dd7da0bf872ca5bd7464e290267c8
Signed-off-by: Jack Pham <jackp@codeaurora.org>