net: ipa: support enhanced channel flow control

IPA v4.2 introduced GSI channel flow control, used instead of IPA
endpoint DELAY mode to prevent a TX channel from injecting packets
into the IPA core.  It used a new FLOW_CONTROLLED channel state
which could be entered using GSI generic commands.

IPA v4.11 extended the channel flow control model.  Rather than
having a distinct FLOW_CONTROLLED channel state, each channel has a
"flow control" property that can be enabled or not--independent of
the channel state.  The AP (or modem) can modify this property using
the same GSI generic commands as before.

The AP only uses channel flow control on modem TX channels, and only
when recovering from a modem crash.  The AP has no way to discover
the state of a modem channel, so the fact that (starting with IPA
v4.11) flow control no longer uses a distinct channel state is
invisible to the AP.  So enhanced flow control generally does not
change the way AP uses flow control.

There are a few small differences, however:
  - There is a notion of "primary" or "secondary" flow control, and
    when enabling or disabling flow control that must be specified
    in a new field in the GSI generic command register.  For now, we
    always specify 0 (meaning "primary").
  - When disabling flow control, it's possible a request will need
    to be retried.  We retry up to 5 times in this case.
  - Another new generic command allows the current flow control
    state to be queried.  We do not use this.

Other than the need for retries, the code essentially works the same
way as before.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>