[PATCH] zd1211rw: Optimized handling of zero length entries in length info

There are a high number of split USB transactions, which contain
only one packet but have a length info field. This patch optimizes
this code by stopping parsing the length info structure if a zero
length field is encountered.

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>