net: sfc: falcon: convert to i2c_new_dummy_device

Move from i2c_new_dummy() to i2c_new_dummy_device(). So, we now get an
ERRPTR which we use in error handling.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>