[SERIAL] Fix UPF_ flag usage with uart_info->flags

The previous change found a bug in the serial SAK handling - because
we were looking for UPF_SAK set in uart_info->flags, we would never
raise a SAK condition.  UPF_SAK is in uart_port->flags.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>