sfc: Clear I2C adapter structure in falcon_remove_nic()

i2c_del_adapter() leaves dangling pointers in the structure.  If we
retry the NIC probe and pass the structure to i2c_add_adapter() again
it will lead to an oops unless we clear it first.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>