serial: 8250: Add missing wakeup event reporting

An 8250 UART configured as a wake-up source would not have reported
itself through sysfs as being the source of wake-up, correct that.

Fixes: b3b708fa

 ("wake up from a serial port")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230414170241.2016255-1-f.fainelli@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>