mmc: sdhci-of-dwcmshc: Prevent illegal clock reduction in HS200/HS400 mode

In the Linux kernel, the following vulnerability has been resolved:

mmc: sdhci-of-dwcmshc: Prevent illegal clock reduction in HS200/HS400 mode

When operating in HS200 or HS400 timing modes, reducing the clock frequency

below 52MHz will lead to link broken as the Rockchip DWC MSHC controller

requires maintaining a minimum clock of 52MHz in these modes.

Add a check to prevent illegal clock reduction through debugfs:

root@debian:/# echo 50000000 > /sys/kernel/debug/mmc0/clock

root@debian:/# [ 30.090146] mmc0: running CQE recovery

mmc0: cqhci: Failed to halt

mmc0: cqhci: spurious TCN for tag 0

WARNING: drivers/mmc/host/cqhci-core.c:797 at cqhci_irq+0x254/0x818, CPU#1: kworker/1:0H/24

Modules linked in:

CPU: 1 UID: 0 PID: 24 Comm: kworker/1:0H Not tainted 6.19.0-rc1-00001-g09db0998649d-dirty #204 PREEMPT

Hardware name: Rockchip RK3588 EVB1 V10 Board (DT)

Workqueue: kblockd blk_mq_run_work_fn

pstate: 604000c9 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)

pc : cqhci_irq+0x254/0x818

lr : cqhci_irq+0x254/0x818

...