iwlwifi: bump FW API to 56 for AX devices

Start supporting API version 56 for AX devices.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200529092401.aabbc5b472ee.I88cb2c3d2d07e62eac3671335ff1fb80b73c5839@changeid