mmc: core: Fix power class config for HS400

Use the correct power class field from the extended CSD register
for HS400 mode as defined in the eMMC5.0 specification.

CRs-fixed: 690341
Change-Id: Ie10e35941fd3c6ee49c686f721bf5af6fcd74862
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>