[PATCH] zd1211rw: cleanups

Add static to 2 internal functions. Thanks goes to Adrian Bunk, who found that.

Also made some modifications to the clear functions:

After a discussion on the mailing list, I implemented this code to
have on the one hand sufficient test in debug mode, but on the
other hand reduce the overhead for structure clearing to a
minimum.

A new macro ZD_MEMCLEAR is introduced, which produces code if
DEBUG is set. Locks are not set anymore for structure clearing,
but in debug mode, there is a verification, that the locks have
not been set.

Finally, removed a misleading comment regarding locking in the disconnect
path.

Signed-off-by: Ulrich Kunitz <kune@deine-taler.de>
Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>