New call-in/-back functions for Controller to do vendor-specific shutdown

A pair of asynchronous call-in and call-back API are added into the Host
Controller Vendor Lib interface (bt_vendor_lib.h). The caller calls this new
call-in function to inform the vendor module to perform vendor-specific
shutdown process. vendor module is responsible for calling call-back function
to notify the caller completion of vendor-specific shutdown process.

Since we don't have any shutdown process, call call-back directly

Change-Id: I30769b0518d9199fb5fe9fef550d2f21b20e7b2e
Signed-off-by: Vishal Mahaveer <vishalm@ti.com>