mmc: cmdq_hci: fix platform device power management reference counting

After issuing a request the usage_count is decremented. After idle time
controller irq is disabled by platform device runtime pm and request
complete irq is not handled.

This change moves decrement of usage_count from the end of issuing request
to the end of request completion.

Change-Id: I1322e0d1ab4ffbf50956fec2921c778e0dcddf36
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>