mmc: core: Let's callers of from mmc_get_ext_csd() do error handling

The callers of mmc_get_ext_csd() need the flexibility to handle errors
themselves, because they behave differently.

Let's clean up mmc_get_ext_csd() with its friends and adopt the error
handling as stated above.

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