mtd: chips: cfi_cmdset_0002: Match ENABLE_VPP()/DISABLE_VPP() calls

This patch is part of a set which fixes unnecessary flash erase and write errors
resulting from the MTD CFI driver turning off vpp while an erase is in progress.
This patch ensures that only those flash operations which call ENABLE_VPP() can
then call DISABLE_VPP(). Other operations should never call DISABLE_VPP().

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>