cxgb4: fix error recovery when t4_fw_hello returns a positive value

Since commit 636f9d37

 ("cxgb4: Add
support for T4 configuration file"), t4_fw_hello may return a positive
value instead of 0 for success. The recovery code tests only for zero
and fails recovery for any other value.

This fix tests for negative error values and fails only on those cases.
Error recovery after an error injection works after this change.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>