Bug 10728: fix log noise generated by subscription-renew.pl

Bug 10728: fix log noise generated by subscription-renew.pl

Error log is showing warnings because mode is undefined
and we are doing string comparisons on it.

Set it to a default value (we were already assigning it a variable
which we were not using) and use the the result in the comparison

To test, after applying the patch:

[1] Use the renew link from the subscription detail page to renew
    a subscription.
[2] Verifying that doing this did not add a warning containing
    'subscription-renew.pl: Use of uninitialized value $mode in string eq '
    to the Apache error log.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>