pwm: lpc18xx: Convert to use dev_err_probe()

This has various upsides:
 - It emits the symbolic name of the error code
 - It is silent in the EPROBE_DEFER case and properly sets the defer reason
 - It reduces the number of code lines slightly

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