efivars: Fix check for CONFIG_EFI_VARS_PSTORE_DEFAULT_DISABLE

The 'CONFIG_' prefix is not implicit in IS_ENABLED().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>