Bluetooth: Separate btmrvl_register_hdev() from btmrvl_add_card()

Move btmrvl hdev registration code out of btmrvl_add_card().
New function btmrvl_register_hdev() is added.

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