host: sdhci-msm: implement get_current_limit() host op

This is needed to get the current capabilities of vdd
regulator that is not managed by SDHCI driver.

Change-Id: Ib41f9892caca7f9d39675c6adf3b5eb8fdf88159
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>
[xiaonian@codeaurora.org: fixed trivial merge conflicts]
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>