pwm: meson: Simplify using dev_err_probe()

Using dev_err_probe() emitting an error message mentioning a return
value and returning that value can be done in a single statement. Make
use of that to simplify the probe part of the driver. This has the
additional advantage to emit the symbolic name for the error instead of
the integer error value.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>