sfc: initialize dynamic sysfs attributes for lockdep

Dynamically allocated sysfs attributes must be initialized using
sysfs_attr_init(), otherwise lockdep complains:
BUG: key <address> not in .data!

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