wifi: mwifiex: mark a variable unused

We need to read a value from the device to wake it, but if it
succeeds we don't really care about it. Mark the variable to
avoid a compiler warning.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220904212910.5d67f55178a1.If0789ab326935896e5886fa06dbb9ef0da6c0b41@changeid