Bluetooth: hci_uart: Fix uninitialized alignment value

Force alignment value to the default one (1 byte) if uninitialized.
This fixes hci_ll serdev driver (alignment = 0) and avoid any further
issues with upcoming drivers.

Signed-off-by: Loic Poulain <loic.poulain@gmail.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>