Use single global lock in the ENA driver

Currently, the driver had 2 global locks - one was sx lock used for
up/down synchronization and the second one was mutex, which was used
for link configuration and timer service callout.

It is better to have single lock for that. We cannot use mutex, as it
can sleep and cause witness errors in up/down configuration, so sx lock
seems to be the only choice.

Callout cannot use sx lock, but the timer service is MP safe, so we just
need to avoid race between ena_down() and ena_detach(). It can be
avoided by acquiring sx lock.

Simple macros were added that are encapsulating implementation of the
lock and makes the code cleaner.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.