pch_uart: Fix return value issue

Currently, occurring line status interrupt,
returned value is not set in interrupt handler function.
As a result, 0 can be returned.

This patch adds setting returned value.

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>