mmc: core: claim host before halt in pm runtime idle

There can be a race condition between runtime idle and incoming
requests. In such a race condition, we can send requests
while the queue is halted.
Claiming the host at the beginning of runtime idle will prevent
such a case.

Change-Id: I8e55f0798543b2de44b37da13f7770889e6fbe5f
Signed-off-by: Maya Erez <merez@codeaurora.org>