spi: Fix tegra20 build with CONFIG_PM=n once again

Commit efafec27 ("spi: Fix tegra20 build with CONFIG_PM=n") already
fixed the build without PM support once.  There was an alternative fix
by Guenter in commit 2bab9409 ("spi: tegra20-slink: Declare runtime
suspend and resume functions conditionally"), and Mark then merged the
two correctly in ffb1e76f ("Merge tag 'v5.15-rc2' into spi-5.15").

But for some inexplicable reason, Mark then merged things _again_ in
commit 59c4e190

 ("Merge tag 'v5.15-rc3' into spi-5.15"), and screwed
things up at that point, and the __maybe_unused attribute on
tegra_slink_runtime_resume() went missing.

Reinstate it, so that alpha (and other architectures without PM support)
builds cleanly again.

Btw, this is another prime example of how random back-merges are not
good.  Just don't do them.  Subsystem developers should not merge my
tree in any normal circumstances.  Both of those merge commits pointed
to above are bad: even the one that got the merge result right doesn't
even mention _why_ it was done, and the one that got it wrong is
obviously broken.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>