Bluetooth: btmrvl: add btmrvl_send_sync_cmd() function

Command preparation code is used multiple times. This patch
separate out this common code and create btmrvl_send_sync_cmd()
function.

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