[electrum] Alllow for downgrading the Trezor firmware

Summary: When the user selects to install a custom firmware, Electrum detects whether it's an official firmware and applies some checks on the header including a version check that prevents from downgrading the firmware. This is not the expected behavior when the firmware is installed from a file and the user should be able to downgrade if desired.

Test Plan: Downgrade an official Trezor firmware using this method.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17842