Fix error handling in the ENA driver and lock drbr_free() call

[mk_semihalf.com]

Summary

Some goto tags were renamed for consistency, and few error handling
routines were reworked.

The drbr_free() must be locked just in case code will change in the
future - for now, it should never be an issue, because drbr is being
flushed in the ena_down() call, and the lock is required only when there
are some mbufs inside.

[mk_semihalf.com]

Update due to conflict with recent fix in the:
D12853 Fix ENA driver error handling in attach and basic style fixes

[byenduri_gmail.com]

Looks good to me.