Bluetooth: btnxpuart: Revert baudrate change in nxp_shutdown

This reverts the change baudrate logic in nxp_shutdown.

Earlier, when the driver was removed, it restored the controller
baudrate to fw_init_baudrate, so that on re-loading the driver, things
work fine.

However, if the driver was removed while hci0 interface is down, the
change baudrate vendor command could not be sent by the driver. When the
driver was re-loaded, host and controller baudrate would be mismatched
and hci initialization would fail. The only way to recover would be to
reboot the system.

This issue was fixed by moving the restore baudrate logic from
nxp_serdev_remove() to nxp_shutdown().

This fix however caused another issue with the command "hciconfig hci0
reset", which makes hci0 DOWN and UP immediately.

Running "bluetoothctl power off" and "bluetoothctl power on" in a tight
loop works fine.

To maintain support for "hciconfig reset" command, the above mentioned fix
is reverted.

Fixes: 6fca6781 ("Bluetooth: btnxpuart: Move vendor specific initialization to .post_init")
Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>