Bluetooth: vhci: validate devcoredump state before side effects

The VHCI force_devcoredump debugfs hook accepts a small test record from
userspace. It validates the requested terminal state only after
registering, initializing and appending a Bluetooth devcoredump.

As a result, an invalid state returns -EINVAL but still leaves queued
devcoredump work behind. With a non-zero timeout field, the rejected
write can still emit a devcoredump after the timeout expires.

Reject unsupported states before allocating the skb or changing the HCI
devcoredump state machine.

Fixes: ab4e4380d4e1 ("Bluetooth: Add vhci devcoredump support")
Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>