iwd: Fix disabling tethering not working for brcmfmac

For Broadcom BCM4356 chipset with brcmfmac driver, changing from AP mode
to station mode returns -EBUSY if the wireless interface is a member of
a bridge.

To resolve the issue, the wireless interface is removed from the tether
bridge before changing the mode rather than after.

Fixes: 648ed549f0ac ("iwd: Add support for tethering")
1 file changed