net/wireless: Fix kernel crash while doing iwlist scan

Remove the code that extracts the information of the rates without
the check for the corresponding bands. Also , this part of the code
is redundant as the following code checks for the same, which also
prevents a possible crash.

Change-Id: I1b4d243cd65f8890f4f7ab4cddf72aad261d7bb0
Signed-off-by: Ming-yi Lin <mylin@codeaurora.org>