Bug 26269: Fix variable name mismatch for cash_register in paycollect

Bug 26269: Fix variable name mismatch for cash_register in paycollect

It appears that through various rebases the variable names in the form
and the controller script have become mismatched.  This patch corrects
the situation and clarifies their intended use.

Test plan:
1/ Turn on cash registers in sysprefs
2/ Define at least 2 cash registers in Admin
3/ Create a manual invoice on a patron
4/ Pay off half of your fee, selecting the first register
5/ Pay off the remaining fee, selecting the second register
6/ Query accountlines.register_id for your two payments
8/ Confirm the two accountlines.register_id's do not match (thus the
passed variable was used)

https://bugs.koha-community.org/show_bug.cgi?id=26469

Signed-off-by: Jessie Zairo <jzairo@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>