net: chelsio: cxgb3: check the return value of pci_find_capability()

The function pci_find_capability() in t3_prep_adapter() can fail, so its
return value should be checked.

Fixes: 4d22de3e ("Add support for the latest 1G/10G Chelsio adapter, T3")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>