Bluetooth: btnxpuart: Move vendor specific initialization to .post_init

This moves change baudrate and power save vendor commands from
nxp_setup() to nxp_post_init().

This also moves the baudrate restore logic from nxp_serdev_remove() to
nxp_shutdown() which ensure baudrate is restored even when HCI dev is
down, preventing baudrate mismatch between host and controller when
device is probed again next time.

In case of removal when the hdev is up and running, we have to call the
shutdown procedure explicitly before unregistering the hdev.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Co-developed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>