Bluetooth: btmtk: add event filter to filter specific event

Add an event filter to filter event with specific opcode to prevent BT
stack from receiving unexpected event.

Event with opcode 0xfc5d is generated when MediaTek's Bluetooth enable
firmware logs and is not expected to be sent to userspace.

Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>