sfc: I2C adapter initialisation fixes

As recommended by Jean Delvare:
- Increase timeout to 50 ms
- Leave adapter class clear so that unwanted drivers do not probe our bus
- Use strlcpy() for name initialisation

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>