Bluetooth: bpa10x: Use h4_recv_buf helper for frame reassembly

The manually coded frame reassembly is actually broken. The h4_recv_buf
helper from the UART driver is a perfect fit for frame reassembly for
this driver. So just export that function and use it here as well.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>