kernel: Fix bug in dynamic alert initialization

k_alert_init() needs to set the "flags" field of its associated
work item to zero, indicating that the work item has not yet
been submitted to the system workqueue. Using the standard work
item initializer macro ensures this is done correctly.

Change-Id: I0001a5920f20fb1d8dc182191e6a549c5bf89be5
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>