mmc: sdhci_am654: Make some symbols static

Fix sparse warnings:

drivers/mmc/host/sdhci_am654.c:192:6: warning: symbol 'sdhci_j721e_4bit_set_clock' was not declared. Should it be static?
drivers/mmc/host/sdhci_am654.c:261:18: warning: symbol 'sdhci_j721e_8bit_ops' was not declared. Should it be static?
drivers/mmc/host/sdhci_am654.c:284:18: warning: symbol 'sdhci_j721e_4bit_ops' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>