ath79: rb91x_nand: fix compilation warning for dev_err

ath79: rb91x_nand: fix compilation warning for dev_err

Fix compilation warning for dev_err in rb91x_nand driver.
Fix compilation warning:
drivers/mtd/nand/raw/rb91x_nand.c:289:25: note: in expansion of macro 'dev_err'
  289 |                         dev_err(dev, "failed to get gpios: %d\n",
      |                         ^~~~~~~
drivers/mtd/nand/raw/rb91x_nand.c:289:61: note: format string is defined here
  289 |                         dev_err(dev, "failed to get gpios: %d\n",
      |                                                            ~^
      |                                                             |
      |                                                             int
      |                                                            %ld
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>