airo: Fix array bounds warning when moving packet payload.

drivers/net/wireless/airo.c: In function ‘encapsulate’:
drivers/net/wireless/airo.c:1421:15: warning: array subscript is above array bounds [-Warray-bounds]
drivers/net/wireless/airo.c: In function ‘decapsulate’:
drivers/net/wireless/airo.c:1509:16: warning: array subscript is above array bounds [-Warray-bounds]

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