cbus: tahvo: Fix build breakage caused by deprecated initializer removal

Use DEFINE_SPINLOCK() for static spinlock initialization as the commit
d04fa5a "locking: Remove deprecated lock initializers" removes the
deprecated SPIN_LOCK_UNLOCKED. Since tahvo_lock is not used elsewhere than
tahvo.c it can be removed from tahvo.h too.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>