[FIX] stock: do not write on picking type

When creating a stock.picking, the related fields picking_type_entire_packs and
picking_type_code were both updated.
This was an issue was required write access on stock.picking.type (warehouse
manager) to create a stock.picking (warehouse user).

The fields were updated as they were present in the view and filled with the
related value. Even if the value was unchanged, it was triggering a write
anyway.

The two fields were in invisible="1" and used only in attrs for other fields.

Making these two fields in readonly does not prevent to use it in the attrs but
avoid to send a value to the server and trigger a useless write.

Fixes #15769