Change argument to return ptr rather than assign to unused local

There was a warning about local variable assigned but not used.
I believe this was actually an implementation and API problem.
The function is called "btm_sec_find_bonded_dev" and returns
a BOOLEAN about whether the search succeeded or not.  I believe
it should also return a pointer to the dev rec if the search
succeeded, but due to an API definition error, it was just
assigning a local rather than returning the pointer ot the rec.

Also add to btm_int.h a function prototype that was missing
and causing a warning about implicit declaration.

Change-Id: Ifcef3afe3acbe12f4c69472fcedd01b268f644dd
Signed-off-by: Mike J. Chen <mjchen@google.com>