My previous commit to iwn(4) broke the scan loop.

My previous commit to iwn(4) broke the scan loop.

The problem happened if we didn't find an AP to connect to after one scan
iteration. The net80211 stack then performs a SCAN -> SCAN transition to
kick off another scan, but the driver treated this transition as a no-op
and remained in SCAN state doing nothing.

To fix this, introduce a flag which keeps track of whether a firmware
scan command is in progress, and start another scan during a SCAN->SCAN
transition if no scan is in progress. Matches what iwm(4) does.
Note that previously (i.e. in 6.2), iwn(4) would always try to start
a new scan regardless of what the firmware was currently doing.

Problem noticed by myself and also by deraadt@
test & ok tb@