mmc: sdhci: Fix potential WARN condition

In case of invalid last_qos_policy, we should
return after WARN_ONCE. Fix it.

Change-Id: If060aa697584f4e88167a561135f882be08e86f6
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Signed-off-by: Bharat Pawar <bpawar@codeaurora.org>
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>