zd1211rw: fix potential array underflow

The first chunk fixes a debugging assert to print a warning about array underflows.
The second chunk corrects a potential array underflow.  I also removed an assert
in the second chunk because it can no longer happen.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>