[PATCH] clk: mediatek: mt7981: feed MSDC from emmc_208m, not emmc_400m

[Brendon C.]

Tested on a GL.iNet GL-X3000 only. The change is not board-specific:
every MT7981 board with eMMC or SD currently runs its bus at half the
rate its device tree requests and will get the requested rate with this
patch, matching MediaTek's SDK kernel and U-Boot. I do not have a
GL-XE3000, CMCC RAX3000M or Cudy TR3000 to confirm on; the test scripts
are in https://github.com/openwrt/openwrt/issues/25960 for anyone who
does.