drivers/net/wireless/iwlwifi/iwl-3945.c Fix type issue on 64bit

Coverity CID: 2265 NEGATIVE_RETURNS

"rate" is of an unsigned type, and the code requires a signed type.
The following patch makes it so.

Signed-off-by: Darren Jenkins <darrenrjenkins@gmailcom>
Signed-off-by: John W. Linville <linville@tuxdriver.com>