Bluetooth: btnxpuart: Enable Power Save feature on startup

This sets the default power save mode setting to enabled.

The power save feature is now stable and stress test issues, such as the
TX timeout error, have been resolved.
commit c7ee0bc8db32 ("Bluetooth: btnxpuart: Resolve TX timeout error in
power save stress test")

With this setting, the driver will send the vendor command to FW at
startup, to enable power save feature.

User can disable this feature using the following vendor command:
hcitool cmd 3f 23 03 00 00 (HCI_NXP_AUTO_SLEEP_MODE)

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>