Bluetooth: btnxpuart: Fix glitches seen in dual A2DP streaming

This fixes a regression caused by previous commit for fixing truncated
ACL data, which is causing some intermittent glitches when running two
A2DP streams.

serdev_device_write_buf() is the root cause of the glitch, which is
reverted, and the TX work will continue to write until the queue is empty.

This change fixes both issues. No A2DP streaming glitches or truncated
ACL data issue observed.

Fixes: 8023dd22 ("Bluetooth: btnxpuart: Fix driver sending truncated data")
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>