mmc: core: Default to generic_cmd6_time as timeout in __mmc_switch()

All callers of __mmc_switch() should now be specifying a valid timeout for
the CMD6 command. However, just to be sure, let's print a warning and
default to use the generic_cmd6_time in case the provided timeout_ms
argument is zero.

In this context, let's also simplify some of the corresponding code and
clarify some related comments.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200122142747.5690-4-ulf.hansson@linaro.org