mmc: sdhci: fix PM Qos code porting error

A porting error is found in function sdhci_post_req,
where pre_req call is misplaced. And function
sdhci_pre_req misses pre_req call. This change
fixes it.

Change-Id: I08aea3b3e45e8c3db5cdac12da1129a12e143787
Signed-off-by: Can Guo <cang@codeaurora.org>