Bluetooth: Fix not clearing signaled flag for conn_change signal

From k_poll_signal_init() documentation:
The poll signal contains a 'signaled' field that, when set by
k_poll_signal(), stays set until the user sets it back to 0. It thus
has to be reset by the user before being passed again to k_poll() or
k_poll() will consider it being signaled, and will return immediately.

Change-Id: I55daac92dd6293ac653fd7fa0f907b0b7fd99d65
Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>