mmc: mmc: Fix partition switch timeout for some eMMCs

mmc: mmc: Fix partition switch timeout for some eMMCs

commit 1c447116d017a98c90f8f71c8c5a611e0aa42178 upstream.

Some eMMCs set the partition switch timeout too low.

Now typically eMMCs are considered a critical component (e.g. because
they store the root file system) and consequently are expected to be
reliable.  Thus we can neglect the use case where eMMCs can't switch
reliably and we might want a lower timeout to facilitate speedy
recovery.

Although we could employ a quirk for the cards that are affected (if
we could identify them all), as described above, there is little
benefit to having a low timeout, so instead simply set a minimum
timeout.

The minimum is set to 300ms somewhat arbitrarily - the examples that
have been seen had a timeout of 10ms but were sometimes taking 60-70ms.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>