iwlwifi: avoid read/write operations if the bus is dead

Recovery takes too much time if the bus is dead (each timeout
is 2000ms, etc.).
Explicitly skip fw dump in this case, as it will result in
garbage data (and might take signifcant time)

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>