task: Don't discard events on mutex contention

On mutex contention, call task_wait_event_mask(), which will wait for
a mutex event without clearing other pending events.

BUG=chrome-os-partner:47918,chromium:435611,chromium:435612
BRANCH=None
TEST=Manual on snoball. Verify samus can successfully negotiate PD power
contract when attached to snoball.

Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Change-Id: I85cd32f2670246da9e4787025390aba2c93f9c36
Reviewed-on: https://chromium-review.googlesource.com/314492
Commit-Ready: Shawn N <shawnn@chromium.org>
Tested-by: Shawn N <shawnn@chromium.org>
Reviewed-by: Alec Berg <alecaberg@chromium.org>
2 files changed