[Cashtab] Use correct type for comparison to dustSats

Summary:
currency.dustSats is a number, 550

remainder is a BigNumber

Both should be a BigNumber to use the gte comparison

Test Plan: npm start, send etoken txs, confirm you get a change output unless change would be dust

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11930