serial: 8250_dw: Add valid clk pointer check

Commit ffc3ae6d "serial: 8250_dw: Enable runtime PM" introduced runtime
PM management, which enables/disables the clk without checking if the clk
is valid. However, this driver allows to be probed without a defined clk,
using clock-frequency, as a fallback.

Therefore, on platforms that are device tree probed using clock-frequency
instead of clk, we get an ugly NULL pointer dereference.

This patch fixes it by simply adding a check before accessing the clk api.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>