b43: Eliminate compilation warning in b43_op_set_key

A recent pull from wireless testing generates the following warning:

   CC [M]  drivers/net/wireless/b43/main.o
 drivers/net/wireless/b43/main.c: In function ‘b43_op_set_key’:
 drivers/net/wireless/b43/main.c:3636: warning: pointer type mismatch
 in conditional expression

This fix was suggested by Johannes Berg <johannes@sipsolutions.net>.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>