Bug 20811: Fix wrong usage of ModBiblio

Bug 20811: Fix wrong usage of ModBiblio

Since
  commit cefa7c21e28b88351ee8ae0dfefb80a515323df9
  Bug 5635: bulkmarcimport new parameters & features

AddBiblio call has been replaced with ModBiblio, but the return values
are different. We should not replace the value of $biblionumber with
what returns this subroutine.

Test plan:
If you are familiar with bulkmarcimport.pl you should know what to test,
I am not.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>