cxgb4: Check for kvzalloc allocation failure

t4_prep_fw doesn't check for card_fw pointer before store the read data,
which could lead to a NULL pointer dereference if kvzalloc failed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>