mwifiex: Ensure the version string from the firmware is 0-terminated

We assume at a few places that priv->version_str is 0-terminated, but
right now we trust the firmware that this is the case with the version
string we get from it.

Let's rather ensure this ourselves and replace the last character with
'\0'.

Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211103201800.13531-4-verdre@v0yd.nl