regulator: core: remove lockdep assert from suspend_prepare

suspend_prepare can be called during regulator init time also, where
the mutex is not locked yet. This causes a false lockdep warning.
To avoid the problem, remove the lockdep assertion from the function
causing the issue. An alternative would be to lock the mutex during
init, but this would cause other problems (some APIs used during init
will attempt to lock the mutex also, causing deadlock.)

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Reported-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>