mmc: sdhci_am654: Make symbol 'sdhci_am654_ops' static

Fixes the following sparse warning:

drivers/mmc/host/sdhci_am654.c:161:18: warning:
 symbol 'sdhci_am654_ops' was not declared. Should it be static?

Fixes: aff88ff23512 ("mmc: sdhci_am654: Add Initial Support for AM654 SDHCI driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>