Bluetooth: bpa10x: Use separate h4_recv_buf helper

When adding the alignment and padding support for H:4 packet processing
for the Nokia driver, it broke the h4_recv_buf usage within bpa10x
driver. To fix this use a separate helper function and placing it into a
dedicated h4_recv.h header file.

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