mmc: core: Use DEFINE_DEBUGFS_ATTRIBUTE instead of DEFINE_SIMPLE_ATTRIBUTE

Fixes coccicheck warning:

drivers/mmc/core/debugfs.c:222:0-23: WARNING: mmc_clock_fops should be
defined with DEFINE_DEBUGFS_ATTRIBUTE

Use DEFINE_DEBUGFS_ATTRIBUTE rather than DEFINE_SIMPLE_ATTRIBUTE
for debugfs files

Fixes: 703aae3d ("mmc: add a file to debugfs for changing host clock at runtime")
Fixes: a04c50aa

 ("mmc: core: no need to check return value of debugfs_create functions")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Link: https://lore.kernel.org/r/1587633319-19835-1-git-send-email-zou_wei@huawei.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>