Fix inconsistent default keyboard delay value used in kwin.

plasma-desktop/keyboard/keyboardmiscsettings.kcfg has the default value
600. When default value (600) is set from kcm, kwin will wrongly use its
own default value in the code (660, which is most likely due to a typo when the
code is initially written).

(cherry picked from commit 8c6ebee3)