pm_runtime_get_sync could fail and thus deserves a check.

The patch adds such a check and return its error code upstream
if it indeed failed.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190324231602.2436-1-kjlu@umn.eduReviewed-by: Mukesh Ojha <mojha@codeaurora.org>