cxgbe(4): Improved error reporting and diagnostics.

"slow" interrupt handler:

• Expand the list of INT_CAUSE registers known to the driver.
• Add decode information for many more bits but decouple it from the rest of intr_info so that it is entirely optional.
• Call t4_fatal_err exactly once, and from the top level PL intr handler.

t4_fatal_err:

• Use t4_shutdown_adapter from the common code to stop the adapter.
• Stop servicing slow interrupts after the first fatal one.

Driver/firmware interaction:

• CH_DUMP_MBOX: note whether the mailbox being dumped is a command or a reply or something else.
• Log the raw value of pcie_fw for some errors.
• Use correct log levels (debug vs. error).

Sponsored by: Chelsio Communications