cxgbe(4): Add a flag to indicate that bits in interrupt cause but not in
interrupt enable are not fatal.

The firmware sets up all the interrupt enables based on run time
configuration, which means the information in the enables is more
accurate than what's compiled into the driver. This change also allows
the fatal bits to be updated without any changes in the driver in some
cases.

MFC after: 1 week
Sponsored by: Chelsio Communications