serdev: make use of printk extension %pe for better error messages

With %pe the symbolic name is printed, so you get

	failure adding device. status -EIO

which is more expressive than the current state

	failure adding device. status -5

.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20191212101649.18126-2-u.kleine-koenig@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>