mtd: rawnand: ams-delta: Fix non-OF build warning

Commit 7c2f66a9 ("mtd: rawnand: ams-delta: Add module device
tables") introduced an OF module device table but wrapped a reference
to it with of_match_ptr() which resolves to NULL in non-OF configs.
That resulted in a clang compiler warning on unused variable in non-OF
builds.  Fix it.

drivers/mtd/nand/raw/ams-delta.c:373:34: warning: unused variable 'gpio_nand_of_id_table' [-Wunused-const-variable]
   static const struct of_device_id gpio_nand_of_id_table[] = {
                                    ^
   1 warning generated.

Fixes: 7c2f66a9 ("mtd: rawnand: ams-delta: Add module device tables")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200919080403.17520-1-jmkrzyszt@gmail.com