[LIBERTAS]: fix oops on the blackfin architecture

Reserve two bytes to align pointer to the IP header.

Signed-off-by: Vladimir Davydov <vladimir.davydov@promwad.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>