Fix a shadow warning in askpassphrasedialog.cpp

Summary:
The toggleShowPassword method defines a local mode variable,
shadowing the AskPassphraseDialog member.
This diff renames the local variable to renderingMode.

Test Plan:

make check

Check the warning is now fixed.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3496