mmc: sdhci_msm: keep a reference to the sdhc host instance

Keep a reference to the sdhc host instance to retrieve all the
necessary data structures in case of a crash for debugging.

Change-Id: I151b160ffbd6a02edea022ae7b970241dedce031
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>