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 <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Acked-by: Paul Cercueil <[email protected]>
Reviewed-by: Alexandre Belloni <[email protected]>
Acked-by: Pratyush Yadav <[email protected]>
Link: https://lore.kernel.org/linux-mtd/[email protected]

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