iwlwifi: mvm: set 512 TX queue slots for AX210 devices

AX210 devices support 256 BA (256 MPDUs in an AMPDU).
The firmware requires that the number of TFDs will be
minimum twice as big as the BA size (2 * 256 = 512).

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>