mmc: core: Drop redundant in-parameter to __mmc_switch()

The use_busy_signal in-parameter is set true by all callers of
__mmc_switch(), hence it's redundant so drop it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Baolin Wang <baolin.wang7@gmail.com>
Tested-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200204085449.32585-5-ulf.hansson@linaro.org