Bluetooth: btmrvl: Fix btmrvl_send_module_cfg_cmd()

Change subcmd parameter from int to u8 to match its use:
btmrvl_send_sync_cmd(priv, BT_CMD_MODULE_CFG_REQ, &subcmd, 1);

Signed-off-by: Petri Gynther <pgynther@google.com>
Reviewed-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>