Bluetooth: hci_uart: add serdev driver support library

This adds library functions for serdev based BT drivers. This is largely
copied from hci_ldisc.c and modified to use serdev calls. There's a little
bit of duplication, but I avoided intermixing this as the ldisc code should
eventually go away.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: linux-bluetooth@vger.kernel.org
Acked-by: Pavel Machek <pavel@ucw.cz>
[Fix style issues reported by Pavel]
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>