net: forcedeth: Replace context and lock check with a lockdep_assert()

nv_update_stats() triggers a WARN_ON() when invoked from hard interrupt
context because the locks in use are not hard interrupt safe. It also has
an assert_spin_locked() which was the lock check before the lockdep era.

Lockdep has way broader locking correctness checks and covers both issues,
so replace the warning and the lock assert with lockdep_assert_held().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Rain River <rain.1986.08.12@gmail.com>
Cc: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>