Bluetooth: btnxpuart: Remove unneeded CONFIG_PM ifdef

The functions are already disabled through the use of pm_ptr() when
CONFIG_PM is disabled, and will be removed from the final linked code
as not needed.

This increases build coverage and allows to drop an #ifdef.

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