mmc: debugfs: Add a restriction to mmc debugfs clock setting

commit e5905ff1 upstream.

Clock frequency values written to an mmc host should not be less than
the minimum clock frequency which the mmc host supports.

Signed-off-by: Yuan Juntao <juntaox.yuan@intel.com>
Signed-off-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>