brcmfmac: rework function picking free BSS index

The old implementation was overcomplicated and slightly bugged in some
corner cases.

Consider following state of BSS-es (limited to 6 for simplification):
drvr->iflist[0]: { bsscfgidx:0, ndev->name:wlan1, }
drvr->iflist[1]:  (null)
drvr->iflist[2]: { bsscfgidx:2, ndev->name:wlan1-1, }
drvr->iflist[3]: { bsscfgidx:3, ndev->name:wlan1-2, }
drvr->iflist[4]:  (null)
drvr->iflist[5]:  (null)
In such case the next AP interface should bsscfgidx 4 (we don't use 1 as
it's reserved for P2P).

With old code the loop iterations were following:
[ifidx = 0] [bsscfgidx = 2] [highest = 2]
[ifidx = 1] [bsscfgidx = 2] [highest = 2] available = true
[ifidx = 2] [bsscfgidx = 2] [highest = 2] bsscfgidx = highest + 1
[ifidx = 3] [bsscfgidx = 3] [highest = 2] bsscfgidx = highest + 1
[ifidx = 4] [bsscfgidx = 3] [highest = 2] available = true
[ifidx = 5] [bsscfgidx = 3] [highest = 2] available = true
There were 2 obvious problems:
1) Having empty BSS at index 1 was resulting in available being always
   set to true, even if we would run out of BSS-es.
2) Calculated bsscfgidx was invalid (3 instead of 4) resulting in driver
   not being able to create the 4th AP interface.

New code is simpler, placed in file where it's really used, handles
running out of free BSS-es and allows using 4 interfaces at the same
time. It also looks for the first free BSS instead of one after the last
in use. It works well with current driver (which doesn't allow deleting
interfaces) and should be future proof (if we ever allow deleting).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

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