iavf: Fix hang during reboot/shutdown

In the Linux kernel, the following vulnerability has been resolved:

iavf: Fix hang during reboot/shutdown

Recent commit 974578017fc1 ("iavf: Add waiting so the port is
initialized in remove") adds a wait-loop at the beginning of
iavf_remove() to ensure that port initialization is finished
prior unregistering net device. This causes a regression
in reboot/shutdown scenario because in this case callback
iavf_shutdown() is called and this callback detaches the device,
makes it down if it is running and sets its state to __IAVF_REMOVE.
Later shutdown callback of associated PF driver (e.g. ice_shutdown)
is called. That callback calls among other things sriov_disable()
that calls indirectly iavf_remove() (see stack trace below).
As the adapter state is already __IAVF_REMOVE then the mentioned
loop is end-less and shutdown process hangs.

The patch fixes this by checking adapter's state at the beginning
of iavf_remove() and skips the rest of the function if the adapter
is already in remove state (shutdown is in progress).

Reproducer:
1. Create VF on PF driven by ice or i40e driver
2. Ensure that the VF is bound to iavf driver
3. Reboot

[52625.981294] sysrq: SysRq : Show Blocked State
[52625.988377] task:reboot state:D stack: 0 pid:17359 ppid: 1 f2
[52625.996732] Call Trace:
[52625.999187] __schedule+0x2d1/0x830
[52626.007400] schedule+0x35/0xa0
[52626.010545] schedule_hrtimeout_range_clock+0x83/0x100
[52626.020046] usleep_range+0x5b/0x80
[52626.023540] iavf_remove+0x63/0x5b0 [iavf]
[52626.027645] pci_device_remove+0x3b/0xc0
[52626.031572] device_release_driver_internal+0x103/0x1f0
[52626.036805] pci_stop_bus_device+0x72/0xa0
[52626.040904] pci_stop_and_remove_bus_device+0xe/0x20
[52626.045870] pci_iov_remove_virtfn+0xba/0x120
[52626.050232] sriov_disable+0x2f/0xe0
[52626.053813] ice_free_vfs+0x7c/0x340 [ice]
[52626.057946] ice_remove+0x220/0x240 [ice]
[52626.061967] ice_shutdown+0x16/0x50 [ice]
[52626.065987] pci_device_shutdown+0x34/0x60
[52626.070086] device_shutdown+0x165/0x1c5
[52626.074011] kernel_restart+0xe/0x30
[52626.077593] __do_sys_reboot+0x1d2/0x210
[52626.093815] do_syscall_64+0x5b/0x1a0
[52626.097483] entry_SYSCALL_64_after_hwframe+0x65/0xca