Floss: initialize HFP debug dump when triggered

When certain conditions are met, the debug dump is in a dirty state that
can contain arbitrary values and cause the process to crash.

This CL initializes the struct so things work as intended.

Bug: 269970706
Test: atest bluetooth_test_gd
Test: btclient > media log
Change-Id: I74f1a72a2fc6d7b7aa1ac540ce9f962866c5176b