mmc: sdhci: Tidy reading 136-bit responses

Read each register only once and move the code to a separate function so
that it is not jammed against the 80 column margin.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>