host: sdhci: fix current caps when there is no host->vmmc

When the regulators are not managed by SDHCI host driver (i.e.,
when host->vmmc and host->vmmcq are absent), get the regulator
current capabilities through a new host op get_current_limit().

Change-Id: I097349ad4d2e8ffbafbeb5fe65910894fe909bd6
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>