Bug 6059: Don't permit to change the quantity when items are created on ordering

Bug 6059: Don't permit to change the quantity when items are created on ordering

If items are created on ordering, the quantity for uncertain prices
should not be editable.

Test plan:
0/ Set AcqCreateItem to "on ordering"
1/ Create a basket and add an order with 1+ items
2/ Mark it as uncertain prices
3/ Go on the uncertain prices page for this vendor
(acqui/uncertainprice.pl?booksellerid=X)
4/ Confirm you are not allowed to update the quantity
5/ Change the price and save
6/ Confirm the quantity is still correct

Set the pref to another value and confirm the behavior has not changed.

NOTE: Tested with 'on placing order' and 'on receiving order'.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit bbb374e76494ae2bad87fbc703e396cebcf8b9f5)
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>