mmc: core: Fix select power class after resume

Use the saved values in card->ext_csd when selecting power class.
By doing this the power class will be selected even if mmc_init_card
is called with oldcard != NULL, which is the case after a suspend/resume.

Today ext_csd is NULL if mmc_init_card is called with oldcard != NULL
and power class will not be selected.

According to the eMMC specification the POWER_CLASS value is reset after
power failure, H/W reset assertion and any CMD0 reset.

Signed-off-by: Fredrik Soderstedt <fredrik.soderstedt@stericsson.com>
Reviewed-by: Johan Rudholm <jrudholm@gmail.com>
Acked By: Girish K S <girish.shivananjappa@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>