mmc: core: Drop open coding in mmc_sd_switch()

The SD_SWITCH (CMD6) is an ADTC type of command with an R1 response, which
can be sent by using the mmc_send_adtc_data(). Let's do that and drop the
open coding in mmc_sd_switch().

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Avri Altman <avri.altman@wdc.com>
Link: https://lore.kernel.org/r/20210504161222.101536-8-ulf.hansson@linaro.org