[qt] Remove assignment in if statement

Summary:
This serves 2 purposes:

1. It is much more readable
2. It works around a discrepancy between clang-format 12.0.0 and 12.0.1

There is no change in behavior.

Test Plan:

ninja check-bitcoin-qt

Send a tx to check the modals work as intended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14591