qlcnic: protect qlicnic_attach_func with rtnl_lock

qlcnic_attach_func requires rtnl_lock to be held.

Cc: Dept-GELinuxNICDev@qlogic.com
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>