mmc: sd: use HIGH_SPEED_BUS_SPEED in mmc_sd_switch_hs()

Use the well defined HIGH_SPEED_BUS_SPEED macro in mmc_sd_switch_hs()
to make code more readable.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200410145643.630b0731@xhacker.debian


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>