iwlwifi: bump FW API to 55 for AX devices

Start supporting API version 55 for AX devices.  We skipped 54 because
it won't be published.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200424182644.5b4628e18203.I626d3b19d84214a83cd92dc4ddf291903b0983aa@changeid