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: b0db0cc2

 ("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>