b43legacy: Fix case where channel status is corrupted

This patch fixes commit 75388acd0cd8 ("add mac80211-based driver for
legacy BCM43xx devices")

In https://bugzilla.kernel.org/show_bug.cgi?id=207093, a defect in
b43legacy is reported. Upon testing, thus problem exists on PPC and
X86 platforms, is present in the oldest kernel tested (3.2), and
has been present in the driver since it was first added to the kernel.

The problem is a corrupted channel status received from the device.
Both the internal card in a PowerBook G4 and the PCMCIA version
(Broadcom BCM4306 with PCI ID 14e4:4320) have the problem. Only Rev, 2
(revision 4 of the 802.11 core) of the chip has been tested. No other
devices using b43legacy are available for testing.

Various sources of the problem were considered. Buffer overrun and
other sources of corruption within the driver were rejected because
the faulty channel status is always the same, not a random value.
It was concluded that the faulty data is coming from the device, probably
due to a firmware bug. As that source is not available, the driver
must take appropriate action to recover.

At present, the driver reports the error, and them continues to process
the bad packet. This is believed that to be a mistake, and the correct
action is to drop the correpted packet.

Fixes: 75388acd0cd8 ("add mac80211-based driver for legacy BCM43xx devices")
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Reported-and-tested by: F. Erhard <erhard_f@mailbox.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200407190043.1686-1-Larry.Finger@lwfinger.net

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