Bluetooth: btnxpuart: Fix driver sending truncated data

This fixes the apparent controller hang issue seen during stress test
where the host sends a truncated payload, followed by HCI commands. The
controller treats these HCI commands as a part of previously truncated
payload, leading to command timeouts.

Adding a serdev_device_wait_until_sent() call after
serdev_device_write_buf() fixed the issue.

Fixes: 689ca16e ("Bluetooth: NXP: Add protocol support for NXP Bluetooth chipsets")
Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>