Bluetooth: btnxpuart: Disable Power Save feature on startup

This sets the default power save mode setting to disabled.
With this setting, this driver will behave like a normal h4 driver.
If user needs to use the power save feature, it can be enabled
using the following vendor command:
hcitool cmd 3f 23 02 00 00 (HCI_NXP_AUTO_SLEEP_MODE)

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>