iwx: re-add sequence numbers for non-AMPDU, preserving previous behaviour

I received a report on freebsd-wireless@ that traffic stopped working.
It seems that although my AX210 is fine with this change, the AX201/22000
hardware/firmware seems to require it.

There's a bit to set in the TX command to force the FW to set the
sequence control. We only set a few flags, and we don't set that.

So, add the sequence number allocation where encryption is performed.

Locally tested:

• AX210, STA mode
• AX201, STA mode

Differential Revision: https://reviews.freebsd.org/D52652
Reviewed by: kevans
Tested by: kevans