sf: Query write-protection status before operating the flash

Do not suggest successful operation if a flash area to be changed is
actually locked, thus will not execute the request. Rather report an
error and bail out. That's way more user-friendly than asking them to
manually check for this case.

Derived from original patch by Chao Zeng.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>