mmc: core: Fetch and decode EXT_CSD from mmc_read_ext_csd()

As a step in cleaning up code around reading/decoding EXT_CSD, convert
the current mmc_read_ext_csd(), to handle both fetching the EXT_CSD
and decoding its data.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>