net: cxgb4: fix return error value in t4_prep_fw

t4_prep_fw goto bye tag with positive return value when something
bad happened and which can not free resource in adap_init0.
so fix it to return negative value.

Fixes: 16e47624

 ("cxgb4: Add new scheme to update T4/T5 firmware")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>