dp_wait_cond() currently returns the number of retries left over which
is hardly an useful information. Convert to returning -ETIMEDOUT when
the wait times out, or 0 (zero) when condition is met before deadline.

Also convert the users of the function to return the error value.
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>