Bluetooth: A2MP: Fix not initializing all members

This fixes various places where a stack variable is used uninitialized.

Taken from https://lore.kernel.org/linux-bluetooth/20200806181714.3216076-1-luiz.dentz@gmail.com/
By Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

Change-Id: I26912bad4c38f5c6078c802d90f1fd25f659ecba