mmc: core: do not retry CMD6 in __mmc_switch()

the response type of CMD6 is R1B, when the first CMD6 gets response
CRC error, do retry may get timeout error due to card may still in
busy state, which cause this retry make no sense.

Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>