mmc: sdhci-brcmstb: Resolve "unused" warnings with CONFIG_OF=n

mmc: sdhci-brcmstb: Resolve "unused" warnings with CONFIG_OF=n

With W=1, we can see this gcc warning:

drivers/mmc/host/sdhci-brcmstb.c:182:34: warning: ‘sdhci_brcm_of_match’ defined but not used [-Wunused-const-variable=]
  182 | static const struct of_device_id sdhci_brcm_of_match[] = {
      |                                  ^~~~~~~~~~~~~~~~~~~

Rather than play around more with #ifdef's, the simplest solution is to
just mark this __maybe_unused.

Fixes: 50bfe185c42a ("mmc: sdhci-brcmstb: Allow building with COMPILE_TEST")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/all/202212060700.NjMecjxS-lkp@intel.com/
Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221205160353.1.I5fa28f1045f17fb9285d507accf139f8b2a8f4b5@changeid
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>