mwifiex: Ignore BTCOEX events from the 88W8897 firmware

The firmware of the 88W8897 PCIe+USB card sends those events very
unreliably, sometimes bluetooth together with 2.4ghz-wifi is used and no
COEX event comes in, and sometimes bluetooth is disabled but the
coexistance mode doesn't get disabled.

This means we sometimes end up capping the rx/tx window size while
bluetooth is not enabled anymore, artifically limiting wifi speeds even
though bluetooth is not being used.

Since we can't fix the firmware, let's just ignore those events on the
88W8897 device. From some Wireshark capture sessions it seems that the
Windows driver also doesn't change the rx/tx window sizes when bluetooth
gets enabled or disabled, so this is fairly consistent with the Windows
driver.

Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211103205827.14559-1-verdre@v0yd.nl