mmc: core: Optimize entering HS400 mode

In the current HS400 selection sequence high speed mode is entered
twice unnecessarily. Skip the first hs selection as it is also done
as part of entering high speed DDR mode.

CRs-fixed: 707453
Change-Id: Id027af977fe416056a6fbef39da1a84e41d89e53
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>