mmc: dw_mmc: avoid write to CDTHRCTL on older versions

Commit f1d2736c (mmc: dw_mmc: control card read threshold) added
dw_mci_ctrl_rd_thld() with an unconditional write to the CDTHRCTL
register at offset 0x100. However before version 240a, the FIFO region
started at 0x100, so the write messes with the FIFO and completely
breaks the driver.

If the version id < 240A, return early from dw_mci_ctl_rd_thld() so as
not to hit this problem.

Fixes: f1d2736c

 (mmc: dw_mmc: control card read threshold)
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: <stable@vger.kernel.org> # v3.13+
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>