rtlwifi: drop unused ppsc->last_wakeup_time

The calculation of ppsc->last_wakeup_time is not y2038-safe, but
the variable is not used at all, so we can simply drop it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>