Bluetooth: mediatek: pass correct size to h4_recv_buf()

We're supposed to pass the number of elements in the mtk_recv_pkts, not
the number of bytes.

Fixes: 7237c4c9 ("Bluetooth: mediatek: Add protocol support for MediaTek serial devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>