mmc: block: Use generic_cmd6_time when modifying INAND_CMD38_ARG_EXT_CSD

The INAND_CMD38_ARG_EXT_CSD is a vendor specific EXT_CSD register, which is
used to prepare an erase/trim operation. However, it doesn't make sense to
use a timeout of 10 minutes while updating the register, which becomes the
case when the timeout_ms argument for mmc_switch() is set to zero.

Instead, let's use the generic_cmd6_time, as that seems like a reasonable
timeout to use for these cases.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200122142747.5690-3-ulf.hansson@linaro.org