sun/niu: add __maybe_unused attribute to PM functions
The upgraded .suspend() and .resume() throw
"defined but not used [-Wunused-function]" warning for certain
configurations.
Mark them with "__maybe_unused" attribute.
Compile-tested only.
Fixes: b0db0cc2f695 ("sun/niu: use generic power management") Reported-by: Stephen Rothwell <sfr@canb.auug.org.au> Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>