mwifiex: Use a define for firmware version string length

Since the version string we get from the firmware is always 128
characters long, use a define for this size instead of having the number
128 copied all over the place.

Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211103201800.13531-2-verdre@v0yd.nl