ray_cs: Fix array bounds warnings.

rx_msg is defined to have a 1 entry array at the end, so gcc warns:

drivers/net/wireless/ray_cs.c: In function ‘rx_authenticate’:
drivers/net/wireless/ray_cs.c:2436:3: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2436:3: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2436:3: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2436:3: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2436:3: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2439:15: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2452:16: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2453:18: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/ray_cs.c:2453:32: warning: array subscript is above array bounds [-Warray-bounds]

Use a zero length array and rename to "ray_rx_msg" to make sure we hit all
of the necessary cases.

Signed-off-by: David S. Miller <davem@davemloft.net>