mmc: core: Drop open coding when preparing commands with busy signaling

Similar code for validating the host->max_busy_timeout towards the current
command's busy timeout, exists in mmc_do_erase(), mmc_sleep() and
__mmc_switch(). Let's move the common code into a helper function.

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/20210504161222.101536-2-ulf.hansson@linaro.org