mmc: card: Fix broken clock gating

The commit '77dacd' misses to add mmc_host_clk_release() in the
completion path of a CQ request i.e., in mmc_blk_cmdq_complete_rq().
Hence, the reference counter of clocks (host->clk_requests) never
becomes 0, preventing the clocks from gating. However, the clocks
are still turned off through other power management features such as
runtime/system suspend.

Change-Id: I0032861b1e5218bdf3c5bed664869c708ce50148
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>