ena: Add locking assertions

ENA silently assumed that ena_up, ena_down and ena_start_xmit routines
should be called within locked context. Driver's logic heavily assumes
on concurrent access to those routines, so for safety and better
documentation about this assumption, the locking assertions were added
to the above functions.

The assertion was added only for the main steps (skipping the helper
functions) which can be called from multiple places including the kernel
and the driver itself.

Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.