Promise from `dispose` method

[Gudahtt]

The dispose method of the keyring calls a dispose function from the Trezor connect that returns a promise, but our method doesn't return that promise. That makes it impossible for the caller to know when this operation has completed. We should return the promise so that it's possible to know when the disposal has completed.

Suggested here: https://github.com/MetaMask/eth-trezor-keyring/pull/161#discussion_r1125740239