fix iwx scan command such that the driver selects an SSID during bgscan

fix iwx scan command such that the driver selects an SSID during bgscan

The driver was asking firmware to send probe requests during background
scans without having any SSID selected, which doesn't make sense.
With that fixed I see faster background scans that no longer display
every AP in range in debug logs, while relevant APs always show up.

This is a possible fix for fatal firmware error 0x20002806 which is
known to be related to bad scan commands and appeared with our upgrade
to the -77 firmware API. Not yet confirmed because it is difficult to
trigger this error on purpose.

Apparently this change also repairs stability issues on AX210.
I have never seen such issues first-hand and have no clue how they
might relate to this change. One plausible theory is that our bogus
bgscan command would trigger bad side-effects in firmware.

Tested:
ax200: jmc@, stsp@
ax210: Laurence Tratt, sf@