cnic: Don't take cnic_dev_lock in cnic_alloc_uio_rings()

We are allocating memory with GFP_KERNEL under spinlock.  Since this is
the only call manipulating the cnic_udev_list and it is always under
rtnl_lock, cnic_dev_lock can be safely removed.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>