sfc: Fix reset vs probe/remove/PM races involving efx_nic::state

sfc: Fix reset vs probe/remove/PM races involving efx_nic::state

We try to defer resets while the device is not READY, but we're not
doing this quite correctly.  In particular, changes to efx_nic::state
are documented as serialised by the RTNL lock, but they aren't.

1. We check whether a reset was requested during probe (suggesting
broken hardware) before we allow requested resets to be scheduled.
This leaves a window where a requested reset would be deferred
indefinitely.

2. Although we cancel the reset work item during device removal,
there are still later operations that can cause it to be scheduled
again.  We need to check the state before scheduling it.

3. Since the state can change between scheduling and running of
the work item, we still need to check it there, and we need to
do so *after* acquiring the RTNL lock which serialises state
changes.

4. We must cancel the reset work item during device removal, if the
state could ever have been READY.  This wasn't done in some of the
failure paths from efx_pci_probe().  Move the cancellation to
efx_pci_remove_main().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>