Bluetooth: btnxpuart: Fix warnings for suspend and resume functions

This fixes the warnings 'nxp_serdev_resume' and 'nxp_serdev_suspend'
defined but not used.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407100518.06Xxq5PK-lkp@intel.com/


Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>