Bluetooth: btrtl: Correct the length of the HCI command for drop fw

The original code did not determine the length value of the HCI command
for drop fw even there's no parameter needed. In this commit, use struct
hci_command_hdr to manage opcode and length. It would be more regular and
more readable.

Suggested-by: Alex Lu <alex_lu@realsil.com.cn>
Signed-off-by: Max Chou <max.chou@realtek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>