mtd: Fix misuses of of_match_ptr()

of_match_ptr() either expands to NULL if !CONFIG_OF, or is transparent
otherwise. There are several drivers using this macro which keep their
of_device_id array enclosed within an #ifdef CONFIG_OF check, these are
considered fine. However, When misused, the of_device_id array pointed
by this macro will produce a warning because it is finally unused when
compiled without OF support.

A number of fixes are possible:
- Always depend on CONFIG_OF, but this will not always work and may
  break boards.
- Enclose the compatible array by #ifdef's, this may save a bit of
  memory but will reduce build coverage.
- Tell the compiler the array may be unused, if this can be avoided,
  let's not do this.
- Just drop the macro, setting the of_device_id array for a non OF
  enabled platform is not an issue, it will just be unused.

The latter solution seems the more appropriate, so let's use it.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/linux-mtd/20220127110631.1064705-1-miquel.raynal@bootlin.com