WiFi manager bugfix: delete the cached psk when changing the password
WiFi manager bugfix: delete the cached psk when changing the password

Previously, the cached psk, which is derived from the password and
passed to wpa_supplicant, was not updated when changing the password.

pazos/koreader,lgeek/koreader,NiLuJe/koreader,mwoz123/koreader,NiLuJe/koreader,Frenzie/koreader,houqp/koreader,robert00s/koreader,mihailim/koreader,Markismus/koreader,Frenzie/koreader,koreader/koreader,koreader/koreader,poire-z/koreader,Hzj-jie/koreader,apletnev/koreader,poire-z/koreader