ena: add locks before each ena_up and ena_down

[mk_semihalf.com]

Summary

Lock only ena_up and ena_down calls in ioctl handler, instead of whole
ioctl. Locking ioctl with sx lock that is sleepable, is not allowed in
some cases, e.g. when multicast options are being changed.
Additional locking was added in detach function to prevent race condition
with ioctl function.