efivars: Fix check for CONFIG_EFI_VARS_PSTORE_DEFAULT_DISABLE

commit ca0ba26fbbd2d81c43085df49ce0abfe34535a90 upstream.

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>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Cc: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>