cfi_flash: Read PPB sector protection from device for AMD/Spansion chips

Patch 66863b05 [cfi_flash: add support for Spansion flash PPB sector
protection] introduced the PPB (Persistent Protection Bit) locking for
Spansion chips. But right now the sector protection status (locked vs
unlocked) is set to unlocked for all sectors upon bootup. The real
sector protection status is ignored.

This patch now reads the current sector protection status and uses
it for these AMD/Spansion flash chips.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Holger Brunck <holger.brunck@keymile.com>
Tested-by: Holger Brunck <holger.brunck@keymile.com>