Bluetooth: btmrvl: get rid of struct btmrvl_cmd

Replace this proprietary structure with the standard one
(struct hci_command_hdr).

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>