mmc: core: Specify timeouts for BKOPS and CACHE_FLUSH for eMMC

The timeout values used while waiting for a CMD6 for BKOPS or a CACHE_FLUSH
to complete, are not defined by the eMMC spec. However, a timeout of 10
minutes as is currently being used, is just silly for both of these cases.
Instead, let's specify more reasonable timeouts, 120s for BKOPS and 30s for
CACHE_FLUSH.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200122142747.5690-2-ulf.hansson@linaro.org