agp/nvidia: Declare value returned by readl() as unused

Fix the compiler warning

  drivers/char/agp/nvidia-agp.c: In function 'nvidia_tlbflush':
  drivers/char/agp/nvidia-agp.c:264:22: warning: variable 'temp' set but not used [-Wunused-but-set-variable]
    264 |         u32 wbc_reg, temp;

by marking the temp variable with __maybe_unused. The affected readl()
is only required for flushing caches, but the returned value is not of
interest.

v2:
	* declare temp as __maybe_unused (Helge)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201114645.15384-6-tzimmermann@suse.de