[Cashtab] Patch validation error preventing send of amounts with less than 8 decimal places

Summary: Validation for correct number of decimal places improperly required 8, instead of no more than 8. This causes the app to error when user attempts to send BCHA amounts with less than 8 decimal placese.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9144