[dwc3] Wait on CMDACT to verify command completion

After submitting a command to the controller, the CMDACT bit is unset by
the controller to confirm that the command is complete. As
`StartNewConfig`, `EpSetConfig` and `EpTransferConfig` do not enable the
CMDIOC interrupt and act on the interrupt, we should block on the
controller setting this bit to ensure that the controller is ready
before submitting subsequent commands.

Bug: 475343371
Test: Validated via test-remote and trace files in bug.
Change-Id: I33c4062fd8c0653ec8d35be096dc8f139f4b4404
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1472761
Reviewed-by: Sam Hansen <hansens@google.com>
Commit-Queue: Prashanth Swaminathan <prashanthsw@google.com>