mmc: core: make mmc_interrupt_hpi() static

Fix the following sparse warning:

drivers/mmc/core/mmc_ops.c:881:5: warning: symbol 'mmc_interrupt_hpi'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Fixes: 55c2b8b9

 ("mmc: core: Re-work the code for eMMC sanitize")
Link: https://lore.kernel.org/r/20200403034727.11879-1-yanaijie@huawei.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>