include/kernel.h: Remove unsupported return values from k_work APIs

Nothing in the code actually returns -EINPROGRESS, and in the case of
k_work_init() I don't see how that can even be done in a reliable way.
Don't claim we do what we don't.  Fixes #14109.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
1 file changed