Regression: bcm4312 lp-phy device not recognized in 4.15 kernel

[Chris Vine]

I have an old netbook with a Broadcom bcm4312 802.11b/g lp-phy
[14e4:4315] (rev 01) wifi device.  It is no longer recognized in kernel
4.15: it works fine with the 4.14 kernel and earlier, with the b43 and
ssb kernel modules.

I cannot give any further information as there are no error messages
logged. The b43 firmware (wl-5.100.138) is not loaded for it, nor are
the ssb/b43/bcma/ mmc_core/mac80211/cfg80211 modules.  The modules can
be loaded by hand without error, but nothing happens and no network
interface is presented.

This is a pretty slow and rubbish netbook but is useful when travelling
to a country which might impound your laptop (no names) and you don't
mind too much losing it.  It would be nice to continue to be able to
use it for that.

Chris