b43: fix DMA error related regression with proprietary firmware

In commit 66cffd6d ("b43: fix transmit failure when VT is switched"),
a condition is noted where the network controller needs to be reset. Note
that this situation happens when running the open-source firmware
(http://netweb.ing.unibs.it/~openfwwf/), plus a number of other special
conditions.

for a different card model, it is reported that this change breaks
operation running the proprietary firmware
(https://marc.info/?l=linux-wireless&m=153504546924558&w=2

). Rather
than reverting the previous patch, the code is tweaked to avoid the
reset unless the open-source firmware is being used.

Fixes: 66cffd6d ("b43: fix transmit failure when VT is switched")
Cc: Stable <stable@vger.kernel.org> # 4.18+
Cc: Taketo Kabe <kabe@sra-tohoku.co.jp>
Reported-and-tested-by: D. Prabhu <d.praabhu@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>